When a cation approaches an anion, the electron cloud of the anion is attracted towards a cation and hence gets distorted. The effect is called polarisation of the anion.
The power of cation to polarise anion is called its polarising power and tendency of the anion to get polarised is called polarisability. The greater is the polarisation produced, more is the neutralisation of the charges and hence the ionic character decreases or the covalent character increases.
The polarising power of the cation and the polarisability of the anion and hence the formation of covalent bond depends on:
1) Small size of the cation
Smaller the cation, greater is its polarising power.
2) Large size of the anion
larger the anion, greater is its polarisability. Covalent character of lithium halides is in the order:
LiI > LiBr > LiCl> LiF
3) Large charge on the cation or anion
Larger the charge on the cation, greater is its polarising power. Hence covalent character increases .
The covalent character of the chlorides is in the order:
NaCl < MgCl2 < AlCl3
Greater the charge on the anion, more easily it gets polarised .
4) Electronic configuration of the cations
If two cations have the same size and charge, then the one with 18 electrons in the outermost shell has greater polarising power than the other with 8 electrons in the outermost shell.
Characteristics of covalent compounds
1) Physical States:The covalent compounds exist in all the three states i.e. solid ,liquid and gases.
2) Crystal structure :The crystal structure of covalent compounds differ from that of ionic compounds.They usually consists of molecules rather than ions.
3) Melting and boiling point: Covalent compounds have low melting and boiling point because the molecules in covalent compounds are held together less rigidly.
4) Solubility: They are generally soluble in organic solvents but insoluble in water and other polar solvents.
5) Electrical conductivity :Since there are no free ions in covalent compounds to conduct electricity, they are bad conductors of electricity.
6) Non-ionic reactions :These compounds are molecular in nature and not ionic, their reactions are molecular and proceed at a much slower rate than those of ionic compounds.
7) Directional characteristics and isomerism: As the atoms in covalent compounds are held together by the shared electrons ,it is rigid and directional.Hence, atoms in a molecule can take up different directional arrangement and thus show structural and stereoisomerism.