Chapter 6th

on Saturday, 24 December 2011
CHAPTER 6
                             CHEMICAL BONDING
                                        MCQs

Q.1      An ionic compound A+ B is most likely to be formed when
            (a)        The ionization energy of A is high and electron affinity of B is low
            (b)        The ionization energy of A is low and electron affinity of B is high
            (c)        Both the ionization energy and electron affinity of B are high
            (d)        Both the ionization energy of A and electron affinity of B are low
Q.2      The number of bonds in nitrogen molecules
            (a)        one  s  and one  p
            (b)        one  s  and  two  p
            (c)        three sigma only
            (d)        two  s  and one  p
Q.3      Which of the following statements is not correct regarding bonding molecular orbitals?
            (a)        bonding molecular orbitals possess less energy than atomic orbitals from which they are formed
            (b)        bonding molecular orbitals have low electron density between the two nuclei
            (c)        every electron in the bonding molecular orbitals contributes to the attraction between atoms
            (d)        bonding molecular orbitals are formed when the electron waves undergo constructive interference
Q.4      Which of the following molecules has zero dipole moment?
            (a)        NH3                                   (b)        CHCl3
            (c)        H2O                             (d)        BF3
Q.5      Which of the hydrogen halides has the highest percentage of ionic character
            (a)        HF                               (b)        HBr
            (c)        HCl                              (d)        HI
Q.6      Which of the following molecules has unpaired electrons in anti–bonding molecular orbitals
            (a)        O2                                      (b)        N2
            (c)        Br2                                     (d)        F2
Q.7      Which of the following involve ionic bonding only?
            (a)        Li3N                             (b)        NaCl
            (c)        NCl3                                  (d)        O2
Q.8      Which of the following involve covalent bonding only?
            (a)        KF                               (b)        KCl
            (c)        CH4                                   (d)        MgCl2
Q.9      Which of the following molecules has a net dipole moment?
            (a)        CO2                                   (b)        CS2
            (c)        SO2                                    (d)        CCl4
Q.10    H2S has a net dipole moment while BeF2 has zero dipole moment, because
            (a)        H2S molecule is linear while BeF2 is angular
            (b)        H2S molecule is angular, while BeF2 molecule is linear
            (c)        Fluorine has more electronegativity than  S
            (d)        Be  is more electronegative than  S
Q.11    Which of the following ions has larger ionic radius?
            (a)        Na+                                        (b)        K+
            (c)        Mg2+                                 (d)        Al3+
Q.12    Which of the following bonds is least polar?
            (a)        H–Se                            (b)        P–Cl
            (c)        H–Cl                            (d)        N–Cl
Q.13    Which one has the least bond angle?
            (a)        NH3                                   (b)        CH4
            (c)        H2O                             (d)        BF3
Q.14    Coordinate covalent bonds are formed by
            (a)        sharing of electrons
            (b)        donation of electrons
            (c)        transference of electrons
            (d)        none of these
Q.15    Which of the following molecules would be expected to have zero dipole moment?
            (a)        H2S                              (b)        PF3
            (c)        TeF6                                  (d)        H2O
Q.16    The bond formed between the elements of low ionization energy and elements of high electron affinity is
            (a)        ionic                            (b)        covalent
            (c)        metallic                                    (d)        coordinate
Q.17    The side ways overlap of two–p orbitals to form a bond is called
            (a)        sigma bond                  (b)        pi (p) bond
            (c)        ionic bond                   (d)        covalent bond
Q.18    The head overlap of p–orbitals of two atoms give rise to bond called
            (a)        sigma bond                  (b)        pi (p) bond
            (c)        ionic bond                   (d)        covalent bond
Q.19    Which element would be the most electronegative element with
            (a)        high ionization energy (IE) and low electron affinity (EA)
            (b)        low ionization energy (IE) and high electron affinity (EA)
            (c)        low ionization energy and low electron affinity
            (d)        high ionization energy and high electron affinity
Q.20    Which element would be the least electronegative element with
            (a)        high I.E. and low E.A. (b)        low I.E. and high E.A.
            (c)        low I.E. and low E.A.  (d)        high I.E. and low E.A.
Q.21    Which of the following substances has the least ionic character in its bond?
            (a)        CCl4                                  (b)        KCl
            (c)        BeCl2                                (d)        MgCl2
Q.22    Which of the following best describes ionization energy?
            (a)        energy needed to remove the most loosely bound electron from its ground state
            (b)        it decreases from left to right across a period
            (c)        it increases down the periodic table
            (d)        it is represented by  x + e ® x + energy
Q.23    Which one of the following characteristics is not usually attributed to ionic substances
            (a)        high melting point        (b)        deform when struck
            (c)        crystalline in solid state
            (d)        well defined three dimensional structure
Q.24    Which of the following bond is less polar?
            (a)        B–Cl                            (b)        C–Cl
            (c)        H–I                              (d)        C–I
Q.25    Which type of the orbital hybridization and geometry is used by the central atom of NH2?
            (a)        sp2 hybridization and trigonal planar
            (b)        sp hybridization and tetrahedral geometry
            (c)        sp2 hybridization and trigonal planar
            (d)        sp3 hybridization and tetrahedral geometry
Q.26    Which of the following compounds has most likely been formed by covalent bonding of atoms
            (a)        CaF2                                  (b)        MgO
            (c)        SiH4                                  (d)        NaCl
Q.27    Identify the compound below which has bonds formed by an overlap of sp and p–orbitals
            (a)        BF3                                    (b)        BeCl2
            (c)        NH3                                   (d)        H2O
Q.28    The most electronegative of these group I element is
            (a)        Na                                (b)        K
            (c)        Li                                 (d)        Cs
Q.29    The type of bonding in HBr is
            (a)        ionic                            (b)        polar covalent
            (c)        non–polar covalent      (d)        coordinate covalent
Q.30    Which of the following statement is not correct
            (a)        sigma bond is weaker than a pi bond
            (b)        sigma bond is stronger than a pi bond
            (c)        double bond is stronger than a single bond
            (d)        double bond is shorter than a single bond
Q.31    Which of the following molecules has a pyramidal structure?
            (a)        CH4                                   (b)        NH3
            (c)        H2O                             (d)        C2H4
Q.32    The bond angle in water is
            (a)        109–5o                             (b)        104.5o
            (c)        107.0o                              (d)        120o
Q.33    During the formation of chemical bond, the potential energy of the system
            (a)        decreases                     (b)        increases
            (c)        does not change                       (d)        none of these
Q.34    H2O molecule has
            (a)        no lone pair                 (b)        one lone pair
            (c)        two lone pairs              (d)        none of these
Q.35    NH3 molecule has
            (a)        no lone pair                 (b)        one lone pair
            (c)        two lone pairs              (d)        three lone pairs
Q.36    In  NH3  the covalent bond formed are due to
            (a)        s–sp overlap                (b)        s–sp2 overlap
            (c)        s–sp3 overlap               (d)        sp2–sp2 overlap
Q.37    Which of the following is largest atom
            (a)        Mg                               (b)        Be
            (c)        Sr                                 (d)        Ca
Q.38    As compared to covalent compounds, ionic compounds generally have
            (a)        low melting points and low boiling points
            (b)        low melting points and high boiling points
            (c)        high melting points and high boiling points
            (d)        high melting points and low boiling points
Q.39    The attractive force that holds atoms together in a molecule is called
            (a)        force of attraction        (b)        electrostatic force
            (c)        bond                            (d)        chemical bond
Q.40    Which of the following bonds will be formed between alkali metals and halogens
            (a)        ionic                            (b)        covalent bond
            (c)        metallic bond               (d)        coordinate covalent bond
Q.41    The bond formed between the atoms by mutual sharing of electrons is
            (a)        ionic                            (b)        coordinate covalent bond
            (c)        covalent                       (d)        metallic
Q.42    A chemical bond formed between two similar atoms is purely
            (a)        ionic                            (b)        covalent
            (c)        metallic                                    (d)        coordinate
Q.43    On the basis of VSEPR model the geometry of BeCl2 is
            (a)        linear                           (b)        trigonal
            (c)        tetrahedral                    (d)        angular
Q.44    On the basis of VSEPR theory, a molecule with three bond pair and no lone pair of electrons will have a structure
            (a)        linear                           (b)        trigonal planar
            (c)        tetrahedral                    (d)        trigonal pyramidal
Q.45    The geometry of NH3 on the basis of VSEPR model is
            (a)        trigonal planar              (b)        trigonal pyramidal
            (c)        tetrahedral                    (d)        linear
Q.46    In which of the following theories the hybridization is considered
            (a)        VSEPR                         (b)        Lewis
            (c)        molecular orbital          (d)        valence bond
Q.47    The angle between 3 sp2 hybrid orbital is
            (a)        90o                                     (b)        120o
            (c)        130o                                  (d)        180o
Q.48    The unhybridized “p” orbital in sp2 hybridization is
            (a)        parallel to sp2               (b)        in the same plane
            (c)        perpendicular to sp2 orbitals
            (d)        out of plane
Q.49    Which of the following theories gives the idea of delocalization of electrons
            (a)        Lewis theory                (b)        VSEPR theory
            (c)        valence bond theory    (d)        molecular orbital theory
Q.50    The tandency of an atom to attract, a shared electron pair towards itself is called
            (a)        electron affinity                       (b)        electronegativity
            (c)        dipole moment             (d)        ionization potential
Q.51    Energy needed to remove an electron from its gaseous atom is called
            (a)        electron affinity                       (b)        ionization energy
            (c)        lattice energy               (d)        electronegativity
Q.52    A bond having partial positive and negative charges is
            (a)        ionic                            (b)        covalent
            (c)        polar covalent              (d)        non–polar covalent
Q.53    A bond formed by the linear overlap of atomic orbitals is called
            (a)        sigma                           (b)        ionic
            (c)        pi                                 (d)        polar
Q.54    Which of the following elements is the most electronegative
            (a)        Li                                 (b)        F
            (c)        O                                 (d)        Cl
Q.55    Some covalent compounds dissolve in water due to
            (a)        hydrolysis                    (b)        hydration
            (c)        hydrogen bonding        (d)        metallic bonding
Q.56    Which of the following compounds will have the lowest boiling point?
            (a)        PH3                                    (b)        ASH3
            (c)        NH3                                   (d)        SbH3
Q.57    Which of the following molecules has a coordinate bond?
            (a)        NH4Cl                          (b)        NaCl
            (c)        HCl                              (d)        AlCl3
Q.58    The half of the difference between the number of electrons in bonding MO and antibonding MO is called
            (a)        molecule order             (b)        bond order
            (c)        proton order                 (d)        electron order
Q.59    The bond order for He2 molecule is
            (a)        zero                             (b)       
            (c)        1                                  (d)        2
Q.60    The bond order for H2 is
            (a)        zero                             (b)       
            (c)        1                                  (d)        1.5
Q.61    The bond order in N2 molecule is
            (a)        zero                             (b)        1
            (c)        2                                  (d)        3
Q.62    The bond order in O2 molecule is
            (a)        1                                  (b)        2
            (c)        3                                  (d)        zero
Q.63    Which one of the following is diamagnetic
            (a)        B2                                       (b)        C2
            (c)        N2                                      (d)        O2
Q.64    Which one of the following molecule is paramagnetic
            (a)        B2                                       (b)        C2
            (c)        N2                                      (d)        F2
Q.65    Which of the following ions is diamagnetic
            (a)        O                               (b)        O
            (c)        O                             (d)        N
Q.66    Pi bond consists of two regions of electron cloud density
            (a)        along the bond axis
            (b)        along and perpendicular to bond axis
            (c)        above and below the bond axis
            (d)        none of these
Q.67    Sigma bond consists of one region of electron density
            (a)        along the bond axis
            (b)        along and perpendicular to bond axis
            (c)        above and below the bond axis
            (d)        none of these
Q.68    The electron cloud density is symmetrical along the bond axis in
            (a)        sigma bond                  (b)        pi bond
            (c)        both sigma and pi bond
            (d)        neither sigma nor pi bond
Q.69    The electron cloud density is not symmetrical along the bond axis in
            (a)        sigma bond                  (b)        pi bond
            (c)        both sigma and pi bond
            (d)        neither sigma nor pi bond
Q.70    Covalent bonds are
            (a)        rigid and directional
            (b)        rigid and non–directional
            (c)        neither rigid nor directional
            (d)        non–rigid and directional
Q.71    Ionic bonds are
            (a)        rigid and directional
            (b)        rigid and non–directional
            (c)        non rigid non directional
            (d)        non–rigid and directional
Q.72    Which of the following statements is correct regarding the covalent compounds
            (a)        covalent compounds do not exhibit isomerism
            (b)        covalent compounds exhibit isomerism
            (c)        covalent compounds are soluble in water
            (d)        covalent compounds are insoluble in non–polar solvents
Q.73    The C–C bond length in ethane (C2H6) is
            (a)        154 pm                                    (b)        133 pm
            (c)        120 pm                                    (d)        105 pm
Q.74    The C–C bond length in ethene (C2H4) is
            (a)        154 pm                                    (b)        133 pm
            (c)        120 pm                                    (d)        105 pm
Q.75    The C–C bond length in ethyne is
            (a)        154 pm                                    (b)        133 pm
            (c)        120 pm                                    (d)        105 pm
Q.76    The atomic radii of the elements have a general trend of fluctuating periodically throughout the
            (a)        group                           (b)        period
            (c)        periodic table               (d)        series
Q.77    Which of the following atom has the shortest atomic radius
            (a)        N                                 (b)        F
            (c)        O                                 (d)        B
Q.78    The half of the single bond length between two atoms in a molecule is called
            (a)        ionic radius of an element
            (b)        covalent radius of an element
            (c)        both ionic and covalent
            (d)        none of these
Q.79    Octet rule is not followed in the formation of
            (a)        CH4                                   (b)        NF3
            (c)        BCl3                                  (d)        H2O
Q.80    Select the atom with the largest ionization energy in the following atoms
            (a)        N                                 (b)        P
            (c)        AS                               (d)        Sb
Q.81    Select the largest atom in the following atoms
            (a)        O                                 (b)        S
            (c)        Se                                (d)        Te
Q.82    Which of the following group of elements on the average has the highest ionization energies
            (a)        IA                                (b)        IIIA
            (c)        IVA                             (d)        VIIIA
Q.83    Molecular orbital theory has
            (a)        the superiority over the VB theory
            (b)        the inferiority over the VB theory
            (c)        neither superiority nor inferiority over VB theory
            (d)        none of these
Q.84    The bond between  H–H  is
            (a)        stronger than the bond between H–Cl
            (b)        weaker than the bond between H–Cl
            (c)        neither stronger nor weaker than the bond between H–Cl
            (d)        none of these
Q.85    In which of the following molecules, the value of bond order in maximum
            (a)        H2                                      (b)        O2
            (c)        N2                                      (d)        Cl2
Q.86    When the S–character of hybridized orbital decreases the bond angle
            (a)        decreases                     (b)        increases
            (c)        does not change                       (d)        becomes zero
Q.87    One of the causes of reactions is that the systems attains the energy state which is of
            (a)        higher in energy           (b)        lower in energy
            (c)        balanced in energy       (d)        equal in energy
Q.88    The increase in the bond energy of a covalent bond is due to
            (a)        electronegativity          (b)        ionization energy
            (c)        polarity                                    (d)        symmetry
Q.89    The polarity of a molecule is expressed by
            (a)        bond strength               (b)        dipole moment
            (c)        bond length                  (d)        shape
Q.90    Dipole moment of H2O is
            (a)        1.85                             (b)        1.82
            (c)        1.87                             (d)        1.83

                                                       ANSWERS
Questions
1
2
3
4
5
Answers
b
b
b
d
a
Questions
6
7
8
9
10
Answers
a
a
c
c
b
Questions
11
12
13
14
15
Answers
b
d
c
b
c
Questions
16
17
18
19
20
Answers
a
b
a
d
c
Questions
21
22
23
24
25
Answers
a
a
b
d
d
Questions
26
27
28
29
30
Answers
c
b
c
b
b
Questions
31
32
33
34
35
Answers
b
b
a
c
b
Questions
36
37
38
39
40
Answers
c
b
c
d
a
Questions
41
42
43
44
45
Answers
c
b
a
b
b
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46
47
48
49
50
Answers
d
b
d
d
b
Questions
51
52
53
54
55
Answers
b
b
a
b
c
Questions
56
57
58
59
60
Answers
c
a
b
a
b
Questions
61
62
63
64
65
Answers
d
b
d
a
c
Questions
66
67
68
69
70
Answers
c
a
a
b
a
Questions
71
72
73
74
75
Answers
c
d
b
b
c
Questions
76
77
78
79
80
Answers
c
b
b
c
a
Questions
81
82
83
84
85
Answers
d
d
a
b
c
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86
87
88
89
90
Answers
a
b
c
b
a


                           SHORT QUESTION WITH ANSWERS
Q.1      Dipole moments of chlorobenzene is 1.70 D and of chlorobenzene is 2.5 D while that of paradichlorbenzene is zero; why?
Ans.
            Benzene has zero dipole moment as it is a symmetrical planar hexagonal molecule. The substitution of benzene ring with two Cl–atoms at the para positions does not add any dipole moment to the benzene as the dipoles created being equal and opposite cancel out each other’s effect. As, there is no such cancellation in monochlorobenzene. These molecules have resultant dipole moments of 1.70 D and 2.5 D respectively.
Q.2      What is covalent bond?
Ans.
            A chemical bond formed by the sharing of a pair of electrons between atoms is called covalent bond.
            H + H  ®  H – H
Q.3      What is meant by a coordinate covalent bond?
Ans.
            A coordinate covalent bond is for a bond formed, when both electrons of the bond are denoted by one atom.
Q.4      What is the difference between a localized p bond and a delocalized one?
Ans.
            In a localized p bond, the electron density is concentrated between the two atoms forming the bond. In a delocalized p bond, the electron density is spread over all the atoms that contribute p orbitals to the net work.
Q.5      How will you differentiate between a polar covalent bond and non–polar covalent bond?
Ans.
            A covalent bond between two dissimilar atoms in which the shared electron pair is not attracted equally by the two atoms and the bonded atoms acquire a partial positive and negative charge is called polar covalent bond.
            A covalent bond between two like atoms such as  H–H,
Cl–Cl, in which shared electron pair is attracted equally by both the atoms is called a non–polar covalent bond.
Q.6      Indicate the hybridization and bond angles associated with each of the following (a) linear (b) tetrahedral (c) trigonal planar.
Ans.
            (a)        Sp. 180o           (b)        Sp3. 109o         (c)        Sp2. 120o
Q.7      What are the similarities and differences between atomic orbitals and molecular orbitals?
Ans.
            Both atomic and molecular orbitals have a characteristic energy and shape each can hold a maximum of the two electrons. Atomic orbitals are localized and their energies are the result of interactions between the subatomic particles in a single atom. Molecular orbitals can be delocalized and their energies are influenced by interactions between electrons on several atoms.
Q.8      Why is the bonding molecular orbital of H2 at lower energy than the electron in a hydrogen atom?
Ans.
            There is a net lowering in energy that accompanies bond formation because the electrons in H2 are strongly attracted to both H nuclei, while in H–atom the electron is attracted by only one nucleus.
Q.9      How many electrons can be placed into each molecular orbital of a molecule?
Ans.
            Two electrons can be placed into each molecular orbital of a molecule.
Q.10    What is meant by an ionic bond?
Ans.
            An ionic bond is a chemical bond formed by the electzostatic attraction between positive and negative ions. The bond formed between two atoms when one or more electrons are transformed from valence shell of one atom to the valence shell of the other. The atom that loses electrons becomes cation and the atom that gains electrons becomes anion. The electronic configurations of both ions are those of noble gas atoms. All metals react with non–metals to form ionic compounds.
            Na(g)  ®  Na+ + e
            Cl(g) + e  ®  Cl(g)
            Na+(g) + Cl(g) ®  NaCl
Q.11    Can a molecule have polar bonds and not a dipole?
Ans.
            Yes a molecule can have polar bonds and not a dipole, if the orientation of the polar bonds in the molecule cancel each other’s effect e.g. CO2.
Q.12    Can a molecule have non–polar bonds only and have a dipole?
Ans.
            No. A molecule which have non–polar bonds only cannot have a dipole.
Q.13    State the difference between a polar bond and a polar molecule.
Ans.
            A polar bond is a covalent bond that exists between two atoms having an electronegativity difference greater than 0.2 a polar molecule results, if one or more polar bonds in a molecule is not balanced by other polar bonds in the
Q.14    According to molecular orbital theory, would Be2 be expected to exist? Explain.
Ans.
            Be2 is not expected to exist, it has bond order of zero and is not energetically favoured over isolated Be atoms.
Q.15    What is meant by bond order?
Ans.
            The bond order is half the difference between the number of bonding electrons and the number of antibonding electrons.
            Bond order  = 
             (No. of bonding electrons – No. of antibonding electrons)
Q.16    What is meant by paramagnetic substance?
Ans.
            A paramagnetic substance is a substance that is attracted by a magnetic field and this attraction is generally the result of unpaired electrons. The more unpaired electrons in a species (substance), the stronger the force of attraction. This type of magnetic behaviour is called paramagnetism.
Q.17    What is meant by a diamagnetic substance?
Ans.
            A diamagnetic substance is a substance that is not attracted by a magnetic field. This property is called diamagnetism. This property generally means that substance has only paired electrons.
Q.18    (a)  what is hybridization at carbon atom in CH4, C2H4 and C2H2.
(b)       The carbon atom in CH4 cannot participate in multiple bonding, whereas that is C2H4 can. Explain.
Ans.
(a)        The hybridization at the carbon atom in CH4 is Sp3, in C2H4 is sp2 and in C2H2 is sp.
(b)        The C atom in CH4 is sp3 hybridized, there are no un–hybridized p orbitals, available for the p overlap required for multiple bonds. In C2H4, the C atom is sp2 hybridized with two  p  atomic orbitals (one on each C atom), available to form the p overlap in the C  =  C double bond.
Q.19    Describe briefly the VSEPR theory.
Ans.
            The VSEPR theory predicts the shape of molecules and ions in which valence shell electron pairs are arranged around the central atom of a molecule or ions in such a way that there is maximum separation so that electron repulsions are minimized and electron nucleus attractions are maximized. Some of these electron pairs are bonding and some are lone pairs. The direction in space of the bonding pairs gives the molecular geometry. A lone pair of electrons occupy more space than a bonding pair. Repulsive forces decrease sharply with increasing interpair angle. They are strong at 90o much weaker at 120o and very weak at 180o. in VSEPR model, each multiple bond is treated as though it were a single electron pair.
Q.20    A lone pair of electrons occupies more space than a bond pair?
Ans:
            A lone pair of electrons occupies more space than bond pair because lone pair is attracted by only one nucleus while bond pair is attracted by two nuclei. Due to less nuclear attraction to lone pair its electronic charge is spread ot more in space than that of bond pair.          

Q.21    Predict the geometry of (a)  BeCl2  (b)  BF3  (c)  SiH4  molecules.
Ans.
(a)        The two bond pairs of electrons in BeCl2 molecule arrange themselves as far apart as possible to minimize the repulsion between them. The only arrangement which can satisfy this condition is linear i.e. at angle of 180o.
            :        Cl –––  –– Cl








(b)        The BF3 molecule containing three bond pairs of electrons is trigonal planar because, this structure gives maximum separation among the three bonding electron pairs.
                 ::: :





(c)        In SiH4 molecule, the electrostatic repulsion between four bonding electron pairs will be minimum, when they are present at corners of a regular tetrahedron making angle 109.5o with each other.
 









Q.22    Describe briefly the valence bond theory.
Ans.
            According to valence bond theory, a covalent bond is formed by pairing of electrons by the overlap of half (partially) filled atomic orbitals of two atoms. The two overlapping orbitals must be valence orbitals, must be half filled and must retain their identities. By overlap means that the electrons of overlapping orbitals share a common region of high electron density, along the line between two nuclei called bond axis.
            This theory explains the bonding in terms of overlapping of atomic orbitals and mixing of atomic orbitals called hybridization. Multiple bond occurs via the overlap of atomic orbitals to give s bonds and p bonds.
Q.23    Why monoatomic cations smaller than their corresponding neutral atoms?
Ans.
            Electrostatic repulsions are reduced by removing an electron from a neutral atom, effective neutral charge increases and therefore the cation is smaller than their corresponding neutral atoms.
Q.24    Why are monoatomic anions larger than their corresponding neutral atoms?
Ans.
            The additional electrostatic repulsion produced by adding an electron to a neutral atom decreases the effective nuclear charge experienced by the valence electrons and increases the size of anions.
Q.25    Why does the size of ions increase as one produced down a column in the periodic table?
Ans.
            Going down a column, valence electrons are further from the nucleus and they experience greater shielding by core electrons. The greater radial extent of the valence electrons outweigh the increase in atomic number. Therefore the size of ions increase as one proceeds down a column.
Q.26    What is an isoelectronic series?
Ans.
            An isoelectronic series is a group of atoms or ions that have the same number of electrons, and thus the same electronic configuration.
Q.27    Why noble gases are most stable?
Ans.
            Noble gases are most stable because their s and p orbitals are completely filled.
Q.28    Why O2 molecule is paramagnetic in nature?
Ans.
            O2 molecule is paramagnetic in nature due to presence of unpaired electrons in its molecule.
Q.29    Why CO2 is non–polar molecule although C–O bond is polar?
Ans.
            Each C–O bond in CO2 is polar. The two bond dipoles in CO2 are equal in magnitude and are exactly opposite in direction. The bond dipoles cancel each other. Therefore, the overall dipole moment of CO2 is zero. Thus CO2 is a non–polar molecule.
            Od  =  C  =  Od+
Q.30    Why H2O is a polar molecule?
Ans.
            H2O is a bent molecule with two polar bonds. Both the bonds are identical, so the bond dipoles are equal in magnitude. Because the molecule is bent, however the bond dipoles do not directly oppose to each other and therefore, do not cancel each other.
            Hence the H2O molecule has an overall dipole moment
(m  =  1.85D), because H2O has dipole moment, it is polar molecule.
Q.31    The melting and boiling points of electrovalent compounds are very high as compared with those of covalent compounds. Explain.
Ans.
            The melting and boiling points of electrovalent compounds are very high, because the ions are tightly packed in the crystal lattice by strong attractive forces and high thermal energy is required to separate them from one another. The atoms of molecules in covalent compounds are held together by weak intermolecular forces and less energy is required to separate the atoms or molecules in a solid or liquid.
Q.32    (a)  Why solid NaCl does not conduct electricity?
(b)       What will happen if electric currents is passed through molten NaCl or its aqueous solution? Explain.
Ans.
(a)        Solid NaCl does not conduct electricity because the oppositely charged ions are held together by strong electrostatic forces and the ions do not free to move.
(b)               However, in the molten NaCl or its aqueous solution the ions because quite free to migrate in an electric field and conduct electricity when passed through them.
Q.33    What is molecular orbital theory?
Ans.
            According to molecular orbital theory, atomic orbitals overlap to form molecular orbitals n atomic orbitals combine to form n molecular orbitals. Half of them are bonding molecular orbitals and half antibonding molecular orbitals. In this combination, the individual atomic orbital character is lost in order to form an entirely new orbital that belongs to whole molecule. The theory successfully explains bond order and paramagnetic property of  O2.
Q.34    In many cases, the distinction between a coordinate covalent and a covalent bond vanishes after bond formation. Explain with the help of an example.
Ans.
            A coordinate covalent bond is not essentially different from other covalent bonds, it involves the sharing of pair of electrons between two atoms. An example is formation of NH4+ ion in which all bonds are identical, so the distinction between a coordinate covalent bond and covalent bond vanishes after bond formation.
            H+ + :NH3 ®    H – ® H+
            or     
Q.35    PF3 is a polar molecule with dipole moment 1.02 D and thus the P–F bond is polar. Si, being in proximity of  P  in the periodic table, it is expected that Si–F bond would also be polar, but  SiF4  has no dipole moment. Explain why it is so?
Ans.
            PF3 has one lone pair of electrons and one P atom lies out of plane of rest of the atom and thus PF3 is pyramidal molecule and has 1.02 D dipole moment and thus the P–F bond is polar.
            SiF4 molecular has tetrahedral shape and the SiF bonds are directed by Symmetrically about the central Si atom. Although Si–F bonds are polar, but all the four bond moments cancel out one another and give SiF4 molecule of zero dipole moment.
Q.36    NaCl is a harder substance at room temperature than glucose explain.
Ans.
            The hardness of substance depends on the strength of the forces between the particles forming a substance. NaCl is an ionic compound and consists of Na+ and Cl ions which are held together by strong electrostatic forces of attraction while glucose consists of molecules which are held together by weak intermolecular forces. Therefore, NaCl is a harder substance at room temperature than glucose.
Q.37    The linear of BeCl2 suggests that central Be atom is sp–hybridized. What type of hybridization a central atom undergoes when the atoms bonded to it are located at the corners of (a) an equilateral triangle and (b)  a regular tetrahedron.
Ans.
(a)        The central atom undergoes Sp2 hybridization when the atoms bonded to it are located at the corners of an equilateral triangle.
(b)        The central atom undergoes Sp3 hybridization when the atoms bounded to it are located at the corners of a regular tetrahedron.
Q.38    A double bond is shorter and stronger than a single bond.
Ans.
            The greater the number of electron pairs in the bond, the shorter and stronger will be the bond because of greater nuclei–electron attractions. A double bond has two shared electron pairs, while a single bond has only one shared electron pair. Thus a double bond has a greater nuclei electron attraction, than a single bond. Therefore, a double bond is shorter and stronger than a single bond.
Q.39    NH3 and H2O can form coordinate covalent bond with H+ but CH4 cannot do so.
Ans.
            NH3 and H2O both have lone pair of electrons on N and O atoms which can donate to a H+ (electron deficient) to from a coordinate covalent bond. In CH4 there is no lone pair of electrons to donate H+ for the formation of coordinate covalent bond. Therefore, NH3 and H2O can form coordinate covalent bond with H+ but CH4 cannot do so.
            H3N: + H+  ®  [H3®  H]+
            H2O + H+  ®  H3O ®  [H2O+  ®  H]+
Q.40    Covalent bond may be non–polar but coordinate covalent bond is always polar.
Ans.
            A covalent bond between two like atoms is always non–polar, whereas between two unlike atoms, it is a polar. In coordinate covalent bond the shared electron pair is denoted by only one of the two bonded atoms. The atom which denotes the electron pair acquires partial positive charge and the atom which accepts the electron pair acquires partial negative charge. Therefore, coordinate covalent bond is always polar. Hence a covalent bond may be non–polar, but coordinate covalent bond is always polar.
Q.41    Molecule of O2 is paramagnetic in nature. Explain.
Ans.
            A substance is paramagnetic, when it has unpaired electrons.
            According to molecular orbital theory, O2 has two unpaired electrons in the degenerate orbitals p2 py and p*2 px. Due to the presence of these two unpaired electrons O2 molecule is paramagnetic.
Q.42    Dipole moment of CO2 is zero but that of CO is o.12 Debye.Why?
Ans:
CO2 is a linear molecule and the tow diploes cancel the effect of each other. In CO there is a single dipole directed from carbon to oxygen and it not cancelled.
Q.43    Why dipole moment of benzene is zero?
Ans:
Benzene is symmetrical planar molecule. It has six C – H there are six dipole moments. All the dipole moments cancel the effect of each other and net value is zero.

Q.44    I.E is index of metallic character why?
Ans:
Metals have loosely held electrons which are delocalized and are responsible for the properties of metals. So, metals have low ionization energies.
Q.45    The abnormality of the bond length and bond strength in HI is less prominent than that of HCl, give reason?
Ans:
Chlorine has higher electronegative than iodine. So, the polarities of HCI and HI bonds are unequal. Therefore, abnormality of bond length and bond strength of HCI is more prominent than HI.
Q.46    How does electro negativity difference decide the nature of ionic bond?
Ans:
When the electro negativity difference between two bonded atoms is 1.7 or move than that, then the bond is said to be ionic, otherwise, covalent. The % age of ionic character is more the 51% when the electro negativity difference is 1.7.
Q.47    Why NH3 and PH3 give coordinate covalent bond?
Ans:
NH3 and PH3 have lone pairs of electrons, which can be donated to H+ to make a coordinate covalent bond. In this way, NH4+ and PH4+ are produced which have perfect tetrahedral structure and all the four bonds have perfectly equal status.

Q.48    Most of the elements of the periodic table attain the electronic configuration of inert gases during bond formation. Justify it
Ans:
Inert gases are not reactive due to complete octet except He, Most of the S- and P- block elements may attain eight electrons in the outermost orbitals they do so either by losing gaining or sharing the electrons.

Q.49    Define octet rule?
Ans:
The tendency of the atoms to attain a maximum of eight electrons in the valence shell is called octet rule.
Q.50    What is an ionic radius?
Ans:    
The ionic radius of an ion is the radius of the ion while considering it to be spherical in shape.

Q.51    What is a covalent radius?
The covalent radius of an element is defined as half of the single bond length between tow similar atoms covalently bonded in a molecule.
Q.52    What is difference between sigma & pi bond?
Ans:
SIGMA BOND:
The bond which is formed by the head to head overlapping called sigma bond. The electron density is present between two nuclei.
Pi BOND:
The bond which is formed by the sideways overlapping of two half filled orbitals. The electron density is present above and below the line joining the two nuclei.


Q.53    Why the size of an atom can not be measured directly?
Ans:
The size of an atom can not be measured directly due following reasons:
(i)There is no sharp boundary of an atom. The probability of finding an electron never becomes zero even at larger distances from the nucleus.
(ii) The electronic probability distribution is affected by neighbouring atoms. For this reason the size of an atom may change from one compound to another.

Q.54    Why E.A of Flourine is less than the expected value?
Ans
Since the size of fluorine is very small when electron is added in the fluorine it is strongly repelled by the already existing electrons. An extra amount of energy is provided to add an electron therefore its electron affinity is less than the expected value.

Q.55    Why sigma bond is stronger than pi bond? Or Why pi bond is more diffused than sigma bond?
Ans:
Sigma bond is more diffused than pi bond due to the linear overlapping of orbitals. Moreover electron density is present between two nuclei which is strongly attracted by two nuclei. While in pi bond electron density is not strongly attracted by two nuclei therefore it is weak than pi bond


Q.56    Define bond length.
Ans.
            The distance between the nuclei of two atoms forming a covalent bond is called bond length. In general it is the sum of the covalent radii of the combined atoms.
Q.57    What is dipole moment? What are its units?
Ans.
            The dipole moment may be defined as the product of electric charge (q) and distance (r) between the two oppositely charged centres. It is vector quantity as it has magnitude and direction. It plays a major role in determining the % age ionic character of a covalent bond and the shapes of molecules.
            The dipole moment is measured in Debye units (D). It is denoted by symbol m.
Q.58    Define bond energy Give its units.
Ans.
            The bond energy is defined as the average amount of energy required to break all bonds of particular type in one mole of substance. It is determined by measuring the heat involved in a chemical reaction.
            It is also defined as the energy required to break Avogadro’s number (6.02 x 1023) of bonds or the energy released when an Avagadro number of bonds are formed. It is a measure of strength of bonds. The bond energy is measured in KJ mol–1.
Q.59    Define the following terms.
(a)        Ionization energy
(b)       Electron affinity
(c)        electronegativity
Ans.   
(a)        Ionization energy:
            The minimum amount of energy required to remove an electron from an atom is called ionization energy.
            It depends upon the atomic size, nuclear charge and shielding effect of electrons.
(b)       Electron Affinity:
            The minimum amount of energy released when an electron is added to an isolated neutral gaseous atom in the lowest energy state to produce an anion is called electron affinity. It is measured in kJ mol–1.
(c)        Electronegativity:
            The tendency of an atom to attract shared pair of electron towards itself is called electronegativity. It is measured in electron volts.

                                        TEXT BOOK EXERCISE
Q.1.     Select the correct statement.
  1. An ionic compound A+B- is most likely to be formed when
a.       the ionization energy of A is high and electron affinity of B is low.
b.      the ionization energy of A is low and electron affinity of B is high.
c.       both the ionization energy of A and electron affinity of B is high.
d.      both the ionization energy of A and electron affinity of B is low.
  1. The number of bonds in nitrogen molecule is
a.       one and one
b.      one and two
c.       three sigma only
d.      two and one
  1. Which of the following statements is not correct regarding bonding molecular orbitals?
a.       Bonding molecular orbitals possess less energy than atomic orbitals from which they are formed.
b.      Bonding molecular orbitals have low electron density between the two nuclei
c.       Every electron in the bonding molecular orbitals contributes to the attraction between atoms.
d.      Bonding molecular orbitals are formed when the electron waves Undergo constructive interference.
  1. Which of the following molecules has zero dipole moment?
a.       NH3                      b.         CHCl3             c.         H2O                 d.         BF3
  1. Which of the hydrogen halides has the highest percentage of ionic character?
a.       HCl                       b.         HBr                 c.         HF       d.         HI
  1. Which of the following species has unpaired electrons antibonding molecular orbitals?
a.                         b.                           c.         B2        d.         F2
Ans:    (i) b     (ii) b    (iii) b   (iv) d   (v) c     (vi) b
Q.2.     Fill in the blanks.
  1. The tendency of atoms to attain maximum of ________ electrons in the valence shell is called completion of octect.
  2. The geometrical shape of SiCl4 and PCl3 can be explained on the has of ________ and ________ hybridizations.
  3. The VSEPR theory stands for ________.
  4. For N2 molecule, the energy of (2p)x orbital is ________ than(2py) orbital.
  5. The paramagnetic property of 02 is well explained on the basis on M.O theory in terms of the presence of _________ electrons in two M.O orbitals.
  6. The bond order of N2 is ________ while that of Ne2 is ________.
  7. The values of dipole moment for CS2 is ________ while for SO2 is ________.
Ans:    (i) eight   (ii) SP3, SP2   (iii) valence shell electron pair repulsion   (iv) greater   (v) un-pairs electron   (vi) tree, zero   (vii) zero, 1.61 D
Q.3.     Classify the statements as true or false. Explain with reasons.
  1. The core of an atom is the atom minus its valence shell.
  2. The molecules of nitrogen (NN) and acetylene (HCCH) are not iso-electronic.
  3. There are four coordinate covalent bonds in ion.
  4. Abond is stronger thanbond and the electrons ofbond are more diffused than bond.
  5. The bond energy of heteroatomic diatomic molecules increases with the decrease in the electro negativities of the bonded atoms.
  6. With increases in bond order, bond length decreases and bond strength increases.
  7. The first ionization energies of the elements rise steadily with increasing atomic number from top to bottom is a group.
  8. A double bond is stronger than a single bond and a triple bond is weaker than a double bond.
  9. The bonds formed between the elements having electro negativity difference more than 1.7 are said to be covalent in nature.
  10. The repulsive force between then two bonding pairs is less than that between the two lone pairs.
  11. The number of covalent bonds an atom can form is related to the number of unpaired electrons it has.
  12. The rules which govern the filling of electrons into the atomic orbitals also govern the filling of electrons into the molecular orbitals.
Ans:    (i) true   (ii) false   (iii) false   (iv) false   (v) false   (vi) true   (vii) false   (viii) true   (ix) false   (x) false   (xi) true   (xii) true
Q.4.     What is chemical bond? Discuss the formation of ionic and covalent bond. How does the E.N differences differentiate between ionic & covenant bonds?
Ans:    //////////////  ????????????????????



Q.5.a.  Define ionization energy and electron affinity. How these qualities change in the periodic table. What factors are responsible for their variation?
b.   Explain, what do you understand by the term electro negativity? Discuss its variations in the periodic table. How does it affect the bond strengths?
Ans:                                        a.       Ionization Energy
            The minimum energy required to remove an electron from gaseous atom to form positive ion is called ionization energy. The process is called ionization. E.g.
            The ionization energy indicates the stability of an isolated atom. Moreover ionization energy is an index to the metallic character.
  1. The elements with low I.E are metals
  2. The elements with high I.E are non-metals
  3. The elements with intermediate I.E values are metalloids.
Factors Influencing Ionization Energies:
            The I.E depends upon following factors
  1. Atomic radius or atomic size
  2. Nuclear charge (Proton number) of atom
  3. Shielding effect of inner electrons
  4. Nature of orbital
Higher Ionization Energies:
            The energy required to remove an electron after the removal of first electron is called second ionization energy. E.g.
            The energy required to remove an electron after the removal of second electron in called third ionization energy. E.g.
            It is fact that 3rd I.E > 2nd I.E > 1st I.E
            The reason is that after the removal of one or more electrons, the remaining electrons are held tightly by nuclear positive charge. Thus further removal of electrons requires more energy.
Ionization Energy and Valency:
            We can guess the valency of an element from gap between first, second, third and higher I.E values. If there is big gap between first and second I.E, then valency of element is one. Similarly if there is big gap between second and third I.E, then valency of element is two. E.g. valency of K is one.
Variation of I.E in Periodic Table:
(a)        I.E in a Group:
            The ionization energy decreases from top to bottom of a group. The reason is that new shells are added from top to bottom of group. Thus valence electrons are at large distance from the nucleus. So force of attraction between nucleus and outer electron decreases. Therefore I.E decreases down the group.
(b)       I.E in a Period:
            The I.E increases from left to right of a period. The reason is that in a period nuclear charge increases one by one but no new shell is added so atomic size decreases. Thus force of attraction between nucleus and outer electron increases. Therefore I.E increases in a period.
Electron Affinity
            The energy released when an electron is added to the partially filled orbital of a gaseous atom to form negative ion is called electron affinity. E.g.
            Electron affinity is the measure of attraction between nucleus of an atom and extra incoming electron.
Factors Influencing Electron Affinity:
            Electron affinity depends upon following factors.
  1. Atomic radius
  2. Nuclear charge
  3. Shielding effect of inner electrons
  4. Nature of orbital
In general electron affinity decreases with increase of atomic radius. The reason is that by increasing distance between nucleus and valence electrons, the force of attraction decreases. Hence E.A decreases with increase of atomic radius.
Variation of E.A in Periodic Table:
  1. E.A in a Group:
When we go from top to bottom in a group the proton number increases and new electronic shells are introduced. So atomic radius increases. Thus force of attraction between nucleus and valence electrons decreases. Hence E.A decrease from top to bottom in a group.
  1. E.A in a Period:
When we go from left to right in a period, the proton number increases one by one but no new electronic shell is added. So atomic radius decreases. Therefore force of attraction between nucleus and valence electrons increases. Hence E.A increases from left to right in a period.
Exceptional Cases:
The electron affinity of fluorine is less than that of chlorine. The reason is that seven valence electrons of fluorine are present in 2s and 2p sub shells. These sub shells have thick electronic cloud. This thick electronic cloud repels the incoming electron. Hence E.A of fluorine is less than that of chlorine.
            b.         ///////////// ????????????????????




Q.6.     Write the Lewis structures for the following compounds:
            (i) HCN           (ii) CCl4          (iii) CS2           (iv)
            (v) NH4OH     (vi) H2SO4       (vii) H3PO4      (viii) K2Cr2O7
            (ix) N2O5         (x) Ag(NH3)2NO3
Ans:    /////////// ??????????????????





Q.7.a.  Explain qualitatively the valence bond theory. How does it differ from molecular orbital theory?
b.   How the bonding in the following molecules can be explained with respect to valence bond theory? Cl2, O2, N2, HF, H2S.
Ans:    (a) Valence Bond Theory:
            According to V.B theory molecule is formed by overlap of two atomic orbitals. In the resulted molecule the atomic orbitals retain their identity. The atomic orbitals are monocentric.
            M.O. Theory:
            According to M.O. theory a molecule is formed by linear combination of atomic orbitals. Here atomic orbitals are buried into each other and lose their identity. Moreover molecular orbitals are Polycentric.
            (b) Cl2:
           
            The Cl2 molecule has a sigma bond due to linear overlap of partially filled orbitals.

            O2 Molecule:
           
            O2 molecule has a double bond (one and one) for other molecules.

Q.8.     Explain VSEPR theory. Discuss the structures of CH4, NH3, SO2, SO3 with reference to this theory.
Ans:    ///////////// ???????????????




See page No. 260, 261, 262, 263, 264
Q.9.     The molecules NF3, BF3 and CIF3 all have molecular formula of the type XF3. But they have different structural formulas. Keeping in view VSEPR theory sketch the shape of each molecule and explain the origin of differing in shapes.
Ans:   
(i)         In BF3, the central atom boron contains three electron pairs. All three pairs are bonding. Thus shape of BF3 is triangular planar. Each angle is of 120o. It is shown in fig.


(ii)       In NF3, the central atom Nitrogen contains four electrons pairs. Three are bonding electrons pairs and one is lone pair of electrons. Thus shape of will be tetrahedral.


(iii)      In ClF3, the central atom Chlorine contains five electron pairs. Three pairs are bonding and two are lone pairs of electrons. Thus according to VSEPR theory the molecule ClF3 has a T-Shaped structure.


Q.10.   The species, NH3,have bond angles of 105o, 107.5o and 109.5o respectively. Justify these values by drawing their structures.
Ans:   
(i)        In, nitrogen atom forms two covalent bonds with two hydrogen atoms. Nitrogen has two lone pairs of electrons. These lone pairs of electrons repel each other. Thus angle decreases from 109.5o to 105o.


(ii)       In, nitrogen forms covalent bonds with three hydrogen atoms. There is one lone pair of electrons on nitrogen atom. The lone pair of electrons on nitrogen atom. The lone pair of electron repels bond pairs of electrons. Thus angledecreases from 109.5o to 107o.


(iii)      In, nitrogen forms three covalent bonds and one coordinate bond. There is no lone pair of electrons on nitrogen. Soion has perfect tetrahedral structure. All bond angles are equal to 109.5o.


Q.11.a Explain atomic orbital hybridization with reference to SP3, SP2 and SP modes of hybridizations for PH3, C2H4 and C2H2. Discuss geometries of CCl4, PCl3 and H2S by hybridization of central atoms.
b.   The linear geometry of BeCl2 suggests that central Be atom is sp-hybridized. What type of hybridization a central atom undergoes, when the atoms bonded to it are corners of (a) an equilateral triangle (b) a regular tetrahedron and (c) triangular bipyramide?

Ans:    ////////////// ????????????????




For C2H2 and C2H2 See page No. 272, 273
a.   Hybridization for PH3:
           In PH3, phosphorus shows SP3 hybridization. Four SP3 hybrid orbitals are resulted. Three SP3 hybrid orbitals form-bonds with 1S orbital of three H-atoms. In fourth SP3 hybrid orbital one lone pair of electrons is present. Due to repulsion between lone pair and bond pairs, the angledecreases from 109.5o to 107o.

            Geometry of CCl4:
           In CCl4, carbon shows SP3 hybridization. Four SP3 hybrid orbitals are formed. They overlap with P-orbitals of four Cl-atoms. So structure of CCl4 is perfect tetrahedral. Each angle is 109.5o.

            Geometry of PCl3:
           In PCl3, phosphorus shows SP3 hybridization. Four SP3 hybrid orbitals are formed. They form three -bonds with P-orbitals of three Chlorine atoms. In the fourth SP3-hybrid orbital a lone pair of electrons is present. Due to lone pair-bond pair repulsion angle decreases from 109.5o to 107.5o

            Geometry of H2S:
           In H2S, sulphur shows SP3 hybridization. Four SP3-hybrid orbitals are formed. Two SP3 orbitals overlap with 1S orbtials of two H-atoms. In the remaining two SP3 orbitals, two lone pairs of electrons are present. Due to lone pair-lone pair repulsions, angledecreases from 109.5o to 104.5o.

b.   (i) When atoms are located at the corners of equilateral triangle, then central atoms SP2-hybridization.
            (ii) When atoms are located at the corners of regular tetrahedron, then central atom shows SP3-hybridization.
            (iii) When atoms are located at the corners of a triangular bipyramide, then central atom shows SP3-hybridization.
Q.12.a Give the basis of the molecular orbital theory and discuss the molecular orbital configurations of the following molecules?
      (i) He2              (ii) N2              (iii) O2             (iv)            (v)
b.   How does molecular orbital theory explain the paramagnetic character of O2,  and  species?

Ans:    ///////////// ?????????????????



 (a)       M.O Diagram of :
            The M.O diagram of is shown below. The electronic configuration of is . The bond order of= = 3

            M.O Diagram of :
            The O.M diagram of is shown below. The electronic can figuration of is as

(b)       The M.O diagram of O2 shows that two unpaired electrons are present in O2. So it is Paramagnetic. The M.O diagram of andshow that they have no unpaired electrons. Thusandare diamagnetic.
Q.13.a Sketch the molecular orbital pictures of
      (i) 2py and 2py                (ii) O2, ,           (iii) He2 and Ne2
b.   Sketch the hybrid orbitals of the species, PCl3, SF6, SiCl4 and.


Ans:    ///////////// ?????????????????



 (a)       Molecular Orbital Picture of Ne2:
            The M.O picture of Ne2 (Neon molecule) is shown below. The bond order = = 0. Because bond order of Neon is zero. So Ne2 molecule does not exist.

(b)       Hybrid Orbital of PCl3:
           Phosphorous shows SP3 hybridization in PCl3. The four SP3 hybrid orbitals are formed. Three formbonds with P-orbitals of three chlorine atoms. In fourth SP3-hybrid orbital lone pair of electrons is present.

            Hybrid Orbital of SF6:
           In SF6, sulphur shows d2SP6 hybridization. Six d2SP6 hybrid orbitals are formed. They all six overlap with P-orbitals of six-fluorine atoms.

            Hybrid Orbital of SiCl4:
           In SiCl4, silicon shows SP3 hybridization. Four SP3 hybrid orbitals are formed. They overlap with four P-orbitals of four Chlorine atoms.

            Hybrid Orbital of :
           In, Nitrogen shows SP3 hybridization. Nitrogen forms three covalent bond with three covalent bond with three H-atoms and one coordinate bond with H+ ion.


Q.14.a Define bond energy. Explain the various parameters which determine its strength.
b.   How do you compare the bond strengths of
            (i)         Polar and non-polar molecules, (ii) andbonds?
c.   Calculate the bond energy of H-Br. The bond energy of H-H is 436 KJ mol-1 and that of Br-Br I s 193 KJ mol-1.

Ans:    ///////////////(a) ???????????????



b.i.       A covalent bond between two alike atoms is called. e.g. Cl-Cl, Br-Br. A covalent bond between two unlike atoms is called polar bond. e.g. , . In a polar bond the shared pair of electrons is slightly shifted towards more electro-negative (E.N) atom. So atoms have partial positive and partial negative charges. Thus atoms are attracted due to extra electrostatic (dipole-dipole) forces. Hence a polar bond is stronger than a non-polar bond.
ii.         A bond formed by head to head or linear overlap of two partially filled orbitals is called-bond. A bond formed by parallel overlap of two partially filled P-orbitals is called-bond. In-bond, overlapping of orbitals is symmetrical on the bond axis. In a-bond overlapping of orbitals is spread above and below the bond axis. So-bond is stronger than a-bond.
c.         Bond energy of= 436 Kj mol-1
            Bond energy of per molecule = = 7.2410-22 Kj
            Bond energy of 1atom of hydrogen = = 3.6210-22 Kj
            Bond energy of= 193 Kj mol-1
            Bond energy of per molecule = = 3.2010-22 Kj
            Bond energy of 1atom of Bromine = = 1.6010-22 Kj
Bond energy of 1molecule of = 3.6210-22 + 1.6010-22 = 5.2210-22Kj
Bond energy of per mole = 5.2210-22  6.021023 = 314.2 Kj mol-1
Q.15.a Define dipole moment. Give its various units. Find relationship between Debye and mc. How does it help to find out the shapes of molecules?
b.   The bond length of H-Br is 1.410-10m. Its observed dipole moment is 0.79D. Find the percentage ionic character of the bond. Unit positive charge = 1.602210-19 c and 1D = 3.33610-30mc.


Ans:    //////////////// (a) ????????????????????


b.         Bond length of HBr, r = 1.410-10m,              = 0.79D
            Unit positive charge, q = 1.602210-19C
            %ionic character = ?
            = qr = 1.602210-191.410-10 = 2.2410-29cm
            = = 6.72 D
            %ionic character of H-Br =
                                                      = = 11.7 %
Q.16.   PF3 is a polar molecular with dipole moment 1.02 D and thus the P-F bond is polar. Si, is in proximity of P in the periodic table. It is expected that Si-F bond would also be polar, but SiF4 has no dipole moment. Explain it?
Ans:   PF3 is a Pyramidal molecule like NH3. All three P-F bonds are polar. Their polarity is not cancelled. So PF3 has a net dipole moment of 1.02D. On other hand SiF4 is a perfect tetrahedral molecule. All four SiF bonds are polar but their polarity is cancelled out. Hence net dipole moment of SiF4 is zero.


Q.17.   Which of the following molecules will be polar or non-polar, sketch the structures and Justify your answer.
            (i) CCl4           (ii) SO3            (iii) SF4            (iv) NF3           (v) PF5
            (vi) SO2           (vii) SF6           (viii) IF7
Ans:   
i.         CCl4:
           The CCl4 molecule is perfect tetrahedral. All CCl bonds are polar. Their polarities cancel each other. Thus net dipole moment is zero. So CCl4 is a non-polar molecule.


ii.        SO3:
           The SO3 molecule is plane triangular. All bonds are polar but their polarities cancel each other. Thus net dipole moment is zero. So SO3 is a non-polar.


iii.       SF4:
           The molecule SF4 is trigonal bi pyramidal. All four SF bonds are polar. Their bond moments do not cancel each other. Hence net dipole moment of SF4 is not zero. Therefore SF4 is a polar molecule.


iv.       NF3:
           The molecule NF3 is trigonal pyramidal. All four NF bonds are polar. Their bond moments do not cancel each other. So net dipole moment is not zero. Hence NF3 is a polar molecule.


v.        PF5:
           The molecule PF5 is trigonal bi pyramidal. All PF bonds are polar. Their bond moments cancel each other. So net dipole moment is zero. Hence PF5 is a non polar molecule.


vi.       SO2:
           The molecule SO2 is a angular Vshaped. Two bond moments do not cancel each other. Thus net dipole moment is 1.6D. Hence SO2 is a polar.


vii.      SF6:
           The molecule SF6 is octahedral. All bond moments cancel one another. Thus net dipole moment is zero. Hence SF6 is a non-polar molecule.


viii.     IF7:
           The molecule IF7 is Pentagonal bi pyramidal. All I-F bond moments cancel each other. So net dipole moment is zero. Hence IF7 is a non-polar.


Q.18.   Classify the statements as true or false. Explain with reasons.
  1. Bond distance is the compromise distance between two atoms.
  2. The distinction between a coordinate covalent bond and a covalent bond vanishes after bond formation in, H3O+ and.
  3. The bond angles of H2O and NH3 are not 109.5o like that of CH4. Although, 0-and N-atoms are sp3 hybridized.
  4. are more diffused than.
  5. The abnormality of bond length and bond strength in HI is less prominent than that of HCl.
  6. Solid sodium chloride does not conduct electricity, but when electric current is passed through molten sodium chloride or its aqueous solution, Electrolysis takes place.
  7. The melting points, boiling points, heat of vaporization and heats of sublimations of electrovalent compounds are higher as compared with those of covalent compounds.
Ans.
  1. When two atoms come close to make a bond, then their attraction increases and P.E decreases. At a certain distance atomic attraction is maximum and energy is minimum. It is compromise distance between two atoms. Here bond formation takes place. It atoms come further closer, then nuclear repulsions takes place and energy of system increases. Here bond formation does not occur. Hence bond distance is the compromise distance between two atoms.
  2. In a covalent bond two atoms provide shared pair of electrons. In a coordinate covalent bonds and one coordinate bond then there is no difference between their bond length and bond energy. E.g. inion all four bonds are taken equally. It is the reason that distinction between covalent and coordinate covalent bond vanishes after their formation.
  3. In NH3 and H2O there is SP3 hybridization like CH4. Four SP3 hybrid orbitals are formed. The angle between SP3 orbitals should be 109.5o. But we know that NH3 has one lone pair and H2O has two lone pairs of electrons. There is repulsion between lone pairs and bonding pairs of electrons. Due to lone pair-bond pair repulsion the bond angles in NH3 and H2O are not 109.5o like CH4 which has no lone pair of electrons.
  4. Sigma bond is formed by head to head (liner) overlap of two partially filled orbitals. Here electron density is symmetrically spread around the bond axis. The Pi-bond formed by parallel overlap of two partially filled P-orbitals. Here electron density is spread above and below the bond axis. It is the reason thatare more diffused than.
  5. Chlorine has smaller size and higher electro negativity than Iodine. Thus HCl has more polarity than HI. Due to this reason abnormality of bond length and bond strength in HI is less prominent than that of HCl.
  6. In solid sodium chloride, the Na+ and Cl- ions have strong electrostatic attractions. Thus they are tightly held and occupy fixed positions. But in molten or solution form, ions become free and move towards opposite electrodes. It is the reason that sold NaCl does not conduct electricity but in molten or solution form NaCl conducts electricity.
In electrovalent compounds, the opposite ions have strong electrostatic attractions. Thus they are tightly held and occupy fixed positions. It is the reason that electrovalent compounds have high melting points, boiling points, heat of vaporization and heat of sublimations as compared with those of covalent compounds.

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