Deviation From Ideal Behavior
Chemical Bonding of Class 11
DEVIATION FROM IDEAL BEHAVIOUR
Still now we have discussed all about those compounds which are either 100 percent ionic or covalent from the Lewis theory i.e. the formation of the compound from the respective elements takes place either by mutual sharing or by complete transfer of electrons resulting either covalent species or ionic species.
Now we know that NaBr is an ionic crystal. Similarly AlBr3 is also an ionic crystal as its formation takes place by complete transfer of electrons. So as both are ionic compounds and as it is a well known fact that ionic compounds have high melting points, so we should expect a high melting point for both. But contrary to our expectations the melting point of NaBr is 775°C and that of AlBr3 is 97.5°C. Similarly CaF2 has got a melting point of 1400°C while that of CaI2 is 575°C. This fact indicates that all those compounds which are ionic from the concept of transfer of electrons are not 100% ionic but have some percent of covalent character induced in it. So it is better to say a compound to be more of ionic rather than totally ionic.
Transition from Ionic to Covalent Character:
In an ideal ionic lattice, the ions are supposed to be hard inelastic spheres just touching each other at equilibrium. But properties of most ionic solids deviate from ideal ionic character because lattice may undergo some distortion. Cations are usually smaller than anions and have higher effective nuclear charge than the latter. The outer electron cloud in a cation is more firmly held than in an anion. Electron cloud in an anion is rather loosely held and may be attracted by the neighbouring cation in the lattice. Hence the anion may not retain its spherical symmetry and may be distorted. Such a phenomenon is known as polarisability of the anion and the capacity of the cation to distort the electron cloud of anion is known as polarization power of the cation. As the anion shift’s slightly its electrons towards cation, the positive charge over cation decreases and negative charge over anion also decreases imparting partial ionic character to the bond. In other words a completely ionic bond becomes partial ionic bond and shifting of electrons towards cation imparts covalent nature to the bond i.e. bond starts behaving like polar covalent bond.
The polarising power of a cation is expressed by an index, φ defined as
where φ is called ionic potential.
Again the extent to which an anion is polarised is given by its polarisability. This is governed by the charge and size of the anion.