The reasons for larger size of anion are:
(i) The negative ion is formed by gain of one or more electrons in neutral atom and the number of electrons increases while the magnitude of nuclear charge remains the same.
(ii) As a result, the same nuclear charge acts on larger number of electrons than were present in the neutral atom. In other words, effective nuclear charge per electron is reduced and the electron cloud is held tightly by the nucleus. This results in increase in size.
Example: Chloride ion is larger in size than chlorine atom.
