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Chemical Bonding

Question 6.28: Why does a bond pair of electrons on an atom occupy lesser space than a lone pair?
Answer: A bonding electron pair is attracted by both nuclei of atoms, while a lone pair (non-bonding electron pair) is attracted by only one nucleus. As lone pair of electrons experiences lesser nuclear attraction, their electronic charges spread out more in space and occupy more space than that for bond pair.

Question 2,29: Why does the forces of repulsion between the electrons follow the following order? Lone pair –lone pair > lone pair–bond pair > bond pair –bond pair.
Answer: A bonding electron pair is attracted by both nuclei of atoms,  whereas a non-bonding by only one nucleus. As non-bonding electron pair experience lesser nuclear attraction, their electronic charges spread out more in space and occupy more space than that for bond pair. As a result lone pair-lone pair repulsion is greater than lone pair – bond pair repulsion, which is in turn greater than bond pair-bond pair repulsion.
Question 2.30: Draw the Lewis structures for the following compounds.
Answer: (i) HCN (ii) CCl4 (iii) CS2 (iv) NH3 (v) AlF3 (vi) NlH4OH (vi) H2SO4 (vii) H3PO4 (viii) K2Cr2O7 (ix) N2O5 (x) Ag(NH3)NO3

Question 2.31: Differentiate between covalent and co-ordinate covalent bonds.
Answer: Covelent bond
1. It is formed by mutual sharing of a pair of electrons between two atoms.
2. It may exist between two similar or dissimilar atoms.
3. It may be single, double or triple bond.
4. It is represented by a short line (-) beween two bonded atoms.
Co-ordinate covelent bond
1. It is formed by donation of a pair of electrons by one of the two bonded atom.
2. It always exist between two dissimilar atom.
3. It is always a single bond.
4. It is always represented by an arrow(->) between two bonded atoms.

Question 2.31: Explain VSEPR theory.
Answer: Basic assumption: The valence electron pairs (lone pair and bond pairs) are arranged around the central atom to remain at a maximum distance in order to minimize the repulsion forces. 
Postulates:
1. Both the lone pairs and bond pairs help in determining the geometry of the molecule.
2. The electron pairs in the valence shell of central atom prefer to stay at maximum distance from each other.
3.The lone pairs occupy more space than bond pair.
4. The magnitude of van der Waal repulsion between the electron pairs is given in the following order: Lone pair–lone pair > lone pair–bond pair> bond pair–bond pair.
5.The two electron pairs of a double bond or three electron pairs of a triple bond occupy more space than the electron pair of a single bond.

Question 2.35: Discuss the structure of CH4, NH3, H2O, BeCl2, BF3, SO2, SO3, with reference to this theory.
Answer: 
Shape of CH4 molecule: In methane, there are four bond pairs around the central carbon atom. They will arrange themselves in tetrahedral geometry in order to minimize the electronic repulsion.
Shape of NH3 molecule: In ammonia, there are three bond pairs and one lone pair around the central nitrogen atom. For maximum separation and minimum repulsion, four electron pairs have tetrahedral arrangement. But the molecular geometry of ammonia is trigonal pyramidal due to three bond pairs. Due to greater repulsion exerted by lone pair, the bond angle is reduced from 109.5ο to 107.5ο. Shape of H2O molecule: In water, there are two bond pairs and two lone pairs around the central oxygen atom. For maximum separation and minimum repulsion, four electron pairs have tetrahedral arrangement. But the molecular geometry of water is angular or bent(v-shaped) due to two bond pairs. Due to greater repulsion exerted by lone pairs, the bond angle is reduced from 109.5ο to 104.5ο.
Shape of BeCl2 molecule: In beryllium chloride, there are two bond pairs around the central beryllium atom. For maximum separation and minimum repulsion, two electron pairs have linear arrangement. Thus, beryllium chloride has a linear geometry with bond angle of 180ο .
Shape of BF3 molecule: In boron triflouride, there are three bond pairs around the central boron atom. For maximum separation and minimum repulsion, three electron pairs have trigonal arrangement. Thus, BF3 has a trigonal planer geometry with each bond angle of 120ο. 
Shape of SO2 molecule: In sulphur dioxide, there are two double bonds and one lone pair around the central sulphur atom. According to VSEPR theory, a double bond behaves like a single bond in determining geometry of molecule. For maximum separation and minimum repulsion, two double bonds and a lone pair have trigonal arrangement. However, the molecular geometry of ammonia is angular or bent(v-shaped) due to two double bonds.
Shape of SO3 molecule: In sulphur trioxide, there are three double bonds around the central sulphur atom. According to VSEPR theory, a double bond behaves like a single bond in determining geometry of molecule. For maximum separation and minimum repulsion, three double bonds have trigonal arrangement. Thus, SO3 has a trigonal planer geometry with each bond angle of 120ο.
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