Question

Describe the theory associated with the radius of an atom as it:

(a) gains an electron
(b) loses an electron

Answer 1

Hint: When an atom gains an electron its atomic radius increases and it forms an anion and the size of an atom will increase while when an atom loses an electron it forms cation and its atomic radius decreases and thus size of the atom also decrease.

Complete step by step answer:
From your chemistry lessons you have learned about the atomic radius and the ionic radius.
Atomic radius is defined as the distance of the outermost shell containing the valence electron from the centre of the nucleus.
Ionic radius is defined as the radius of ions from the centre of the nucleus. The ion may be cation or anion.
Let us describe both cases given in the question one by one,

(a) When an atom gains an electron its atomic radius increases and it forms an anion. In this case the atomic radius will be smaller as compared to the radius of anion because here the nuclear charge will remain the same only the no. of electrons will increase and the anion will be larger than the parent atom. So, the force of attraction between the nucleus and the electron will decrease and thus the atomic radius and the size of the atom will increase. For example the atomic radius of F is 64 pm and the ionic radius of ${{F}^{-}}$ is 136pm.

(b) When an atom loses an electron its atomic radius decreases and it forms cation. In this case the atomic radius will be larger than the ionic radius because the no. of the electrons will decrease by 1 but the nuclear charge will remain the same and the cation will be smaller than the parent atom. Hence the force of attraction between the nucleus and the electron increases and thus the atomic radius and the size will decrease. For example the atomic radius of Na is 186pm while the ionic radius of $N{{a}^{+}}$ is 95pm.
Thus atomic radius increases as it gains electrons and atomic radius decreases as it loses electrons.

Note: When the no. of electrons increases in an anion the repulsion between the electron also increases and this decrease the effective nuclear charge and size of an atom increases whereas in cation the interelectronic repulsion is less due to less no. of electron and thus its effective nuclear charge increases and size of an atom decreases.