Question

Which changes occur as a cadmium atom, Cd, becomes a cadmium ion, Cd2+?

(1) The Cd atom gains two electrons and its radius decreases.
(2) The Cd atom gains two electrons and its radius increases.
(3) The Cd atom loses two electrons and its radius decreases.
(4) The Cd atom loses two electrons and its radius increases.

Answer 1

The answer is (3). To form the cation (positive ion), Cd must lose negative charge, so two electrons are lost. This loss in electrons also increases the effective nuclear charge/ reduces electron shielding. In other words, with two less electrons to hold, the nucleus exerts a stronger attractive force on the remaining electrons, so the electron cloud is pulled closer to the nucleus. The radius therefore decreases.