Describe the theory associated with the radius of an atom as it gains on electron.

Gain of  the electron: When a neutral atom gains one electron to form an anion, its radius increases. In the anion, the nuclear charge is the same as that of the parent atom but the number of electrons has increased. Since same nuclear charge now attracts the greater number of electrons, therefore, a force of attraction of the nucleus on the electrons of all the shells decreases i.e. effective nuclear charge per electron decreases in the anion and hence electron cloud expands. The relative sizes of the chlorine atom and chloride ion have been shown.