Ions are formed when an atom loses or gains electrons. When an atom loses an electron it forms a cation and when it gains an electron it becomes an anion. The Ionic radius can be described as the distance between the nucleus of an ion and the outermost shell of the ion.

The atomic size of a cation will be smaller than that of the parent atom. An anion is relatively larger in size than its parent atom. This is because when an atom gains electrons the total number of electrons increases which tends to create more repulsion between electrons and thus overshadows the net effective nuclear charge.

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