Atomic Radius Changes in Ions: Understanding Cations and Anions

The radius of an atom changes when it becomes an ion because the number of electrons and the overall charge of the atom are altered.

When an atom loses or gains electrons to become an ion, the number of electrons no longer balances the number of protons in the nucleus. For example, when an atom loses an electron, it becomes a positively charged ion known as a 'cation'. Since there are now more positively charged protons than negatively charged electrons, the effective attraction between the electrons and the nucleus increases. This leads to a contraction of the electron cloud, resulting in a smaller radius compared to the neutral atom.

On the other hand, when an atom gains an electron, it becomes a negatively charged ion known as an 'anion'. The additional electron increases the repulsion between the negatively charged electrons, causing the electron cloud to expand. This results in a larger radius compared to the neutral atom.

In summary, the radius of an atom changes when it becomes an ion due to the alteration in the number of electrons and the overall charge, leading to a contraction or expansion of the electron cloud.