Describe the theory associated with the radius of an atom as it:
(a) Gain an electron, (b) Loses an electron
When a neutral atom gains an electron to form an anion, the number of electrons in the anion increases whereas its nuclear charge remains the same as that of the parent atom. Since the same nuclear charge now attracts larger number of electrons, therefore, the force of attraction of the nucleus on the electrons of all shells decreases. This means that the effective nuclear charge decreases and the expression of the electron cloud occurs. Because of this, the distance between the centre of the nucleus and the last shell increases thereby increasing the ionic radius.
(b) When a neutral atom losses an electron to form a cation, the number of electrons in the cation decreases, whereas its nuclear charge remains the same as-that of the parent atom. Since the same nuclear charge now attracts lesser number of electrons, therefore the force of attraction of the nucleus on the electrons of all shell increases. This means that the effective nuclear charge increases and the contraction of the electron cloud occurs. Because of this distance between the centre of the nucleus and the last shell decreases thereby decreasing the ionic radius.