Ionic Radius

Ionic radius may be defined as the, “effective distance from the nucleus of the ion to the point up to which it has an influence in the Ionic bond.”

The equilibrium distance between the nuclei of the two adjacent ions can be determined by X-ray analysis. Knowing the ionic radius of one of the ion, the ionic radius of other can be calculated.

The study of Ionic radii leads to two very important generalizations;

(i).  The size of cation is smaller as compared to that of present atom.

(ii). The size of anion is larger as compared to that of present atom.

(I).  RADIUS OF CATION IS SMALLER THAN PRESENT ATOM

A cation is formed, by loss of one or more electrons from gaseous atom. Thus, the nuclear charge (i.e. number of protons) remains same as that in parent atom, but the number of electrons becomes less in valence shell. As a result, the force of attraction on these electrons by nuclear charge increases due to increase in effective nuclear charge per electron. This causes shrinking of shell and hence size decreases.

(II). RADIUS OF ANION IS LARGER THAN PARENT ATOM

An Anion is formed by gain of one or more electrons by the gaseous atom. Thus the nuclear charge remains same (i.e. number of protons), but the number of electrons increases in valence shell. As a result, the force of attraction on these electrons decreases due to decrease in effective nuclear charge per electron. This causes increase in size.