Question

Arrange the following ions in order of their increasing ionic radii: Li⁺, Mg²⁺, K⁺, Al³⁺.

Answer 1

(i) The ionic radius of any cation increases as the number of energy shells increases and decreases as the magnitude of the positive charge increases. 

(ii) Mg²⁺ (1s² 2s² sp⁶) and Al³⁺ (1s² 2s² 2p⁶) are isoelectronic ions and each one of these has two energy shells. Since the positive charge on Al³⁺ is higher than that on Mg²⁺, therefore ionic radius of Al³⁺ is lower than that of Mg²⁺. 

(iii) Since, K⁺ (1s² 2s² 2p⁶ 3s² 3p⁶) has three shells and Mg²⁺ and Al³⁺ have two shells each, therefore, ionic radius of K⁺ is the largest followed by Mg²⁺ and then Al³⁺. 

(iv) Now Li⁺ (1s²) has one shell and +1 charge but Al³⁺ (1s² 2s² 2p⁶) has two shells and +3 charge. Since the increase in the ionic radius of Al³⁺ due to the presence of two shells is more than counter balanced by the decrease in its size due to an increase in charge from +1 in Li⁺ to +3 in Al³⁺, therefore, the ionic radius of Al³⁺ is lower than that of Li⁺. Thus, the ionic radii of these four ions increase in the order : Al³⁺ < Li⁺ < Mg²⁺ < K⁺.