Describe the theory associated with the radius of an atom as it :
(a) gains an electron (b) loses an electron
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Solution
(a) When an atom gains an electron, it forms anion. The atomic radius is smaller than the radius of the anion. This is because the number of electrons has increased but the nuclear charge remains the same. Hence, the electrons are less tightly held by the nucleus due to which the size increases.
(b) When an atom loses an electron, it forms a cation. The atomic radius is larger than the ionic radius. The nuclear charge remains the same but the number of electrons decreases by 1. Hence, the effective nuclear charge per electron increases. The attraction between the nucleus and the valence electrons increases. Due to this, the radius decreases.