Consider the following species: `P^(3-), S^(2-), CI^(-), K^(+), Ca^(2+)` and `Sc^(3+)`
(i) What is common in them?
(ii) Arrange them in the order of decreasing ionis radii.

Answer

(i) Each one of these ions contains `18` electrons and hence all are isoelectronic ions.
(ii) The ionic radii of isoelectronic ions decreases with the increases in the magnitude of the nuclear charge. For example, consider the isoelectronic ions :`P^(3-), S^(2-), CI^(-), K' Ca^(2+)` and `Sc^(3+)`. All these ions have `18` electrons but their nuclear charges increase in the order: `P^(3-) (+15), S^(2-) (+16), CI^(-) (+17), K^(+) (+19) , Ca^(2+) (+20)`and `Sc^(3+)(+21)`. Therefore, their ionic radii decreases in the order: `P^(3-) gt S^(2-) gt CI^(-) gt K^(+) gt Ca^(2+) gt Sc^(3+)`.