of transition metals, is more polarising than the one with a noble gas configuration, ns np , typical of alkali and alkaline earth metal cations. The cation polarises the anion, pulling the electronic charge toward itself and thereby increasing the electronic charge between the two. This is precisely what happens in a covalent bond, i.e., buildup of electron charge density between the nuclei. The polarising power of the cation, the polarisability of the anion and the extent of distortion (polarisation) of anion are the factors, which determine the per cent covalent character of the ionic bond.
. The Valence Shell Electron Pair Repulsion (VSEPR) Theory As already explained, Lewis concept is unable to explain the shapes of molecules. This theory provides a simple procedure to predict the shapes of covalent molecules. Sidgwick Dipole moments of some molecules are shown in Table .
. Just as all the covalent bonds have some partial ionic character, the ionic bonds also have partial covalent character. The partial covalent character of ionic bonds was discussed by Fajans in terms of the following rules: Table . Dipole Moments of Selected Molecules Type of Molecule Example Dipole Moment, µ (D) Geometry Molecule (AB) HF HCl HBr Hl H .
. . . linear linear linear linear linear Molecule (AB ) H O H S CO .
. bent bent linear Molecule (AB ) NH NF BF . . trigonal-pyramidal trigonal-pyramidal trigonal-planar Molecule (AB ) CH CHCl CCl .
tetrahedral tetrahedral tetrahedral and Powell in , proposed a simple theory based on the repulsive interactions of the electron pairs in the valence shell of the atoms. It was further developed and redefined by Nyholm and Gillespie ( ). The main postulates of VSEPR theory are as follows: The shape of a molecule depends upon the number of valence shell electron pairs (bonded or nonbonded) around the central atom. Pairs of electrons in the valence shell repel one another since their electron clouds are negatively charged.
These pairs of