Chemical Bonding and Molecular Structure

4.44.  (c) R is an incorrect statement. Electrons placed in a bonding molecular orbital tend to hold the nuclei together and stabilise a molecule. Therefore, a bonding molecular orbital alwayspossesses lower energy than either of the atomic orbitals that have combined to form it.In contrast, the electrons placed in the antibonding molecular orbital destabilise the molecule.

This is because the mutual repulsion of the electrons in this orbital is more than the attraction between the electrons and the nuclei, which causes a net increase in energy.

4.43.  (d) Basically the strength of a bond depends uponthe extent of overlapping. In case of sigma bond, the overlapping of orbitals takes place to alarger extent. Hence, it is stronger as comparedto the pi bond where the extent of overlappingoccurs to a smaller extent. Further, it isimportant to note that in the formation ofmultiple bonds between two atoms of amolecule, pi bond (s) is formed in addition to asigma bond.

4.42.  (a) In tetra-atomic molecule like BF3, the dipole moment is zero although the B – F bonds are oriented at an angle of 120^o to one another, the three bond moments give a net sum of zero as the resultant of any two is equal and opposite to the third

4.41.  (b) Reason do not explain the assertion, although both are correct statements independently. The dipole moment in case of BeF₂ is zero because the two equal bond dipolespoint in opposite directions and cancel theeffect of each other.

4.40. Bond order is defined as one half the difference between the number of electrons present in the bonding and the antibonding orbitals i.e.

Bond order (B.O.) = ½ (Nb–Na)

4.39. When hydrogen is attached with highly electronegative element in a covalent bonding the electrons of the covalent bond are shifted towards the more electronegative atom. Thus, a partially positively charged hydrogen atom forms a bond with the other more electronegative atom. This bond is known as a hydrogen bond. Hydrogen bond is stronger than the van there Waals forces.

4.38. P has ground state valence shell electronic configuration 3s²3p³ and the first excited state valence shell electronic configuration is 3s¹3p³3d¹. P undergoes sp³d hybridization and has trigonal bipyrami geometry. Because axial bond pairs suffer more repulsive interaction from the equatorial bond pairs, therefore axial bonds have been found to be slightly longer and hence slightly weaker than equatorial bond.

4.37. Molecular orbitals are represented by wavefunctions. A positive sign in an orbital indicates a positive wavefunction while a negative sign in an orbital shows a negative wavefunction.