Explain why cations are smaller and anions larger in radii than their parent atoms.

(N/A) Radius of positive ion $(cation)$: An atom loses electrons to form a positive ion. The size of the positive ion is smaller than its parent atom.
This is because the positive ion has fewer electrons than the parent atom, while the nuclear charge remains the same. Consequently, the remaining electrons experience a greater effective nuclear charge, pulling them closer to the nucleus.
Example: The radius of a sodium atom is $186 \ pm$, whereas the radius of a $Na^{+}$ ion is $95 \ pm$.
Radius of negative ion $(anion)$: When an atom gains an electron, it becomes a negative ion. The size of the negative ion is larger than its parent atom.
This is because when one or more electrons are added to the parent atom, the inter-electronic repulsion increases, and the effective nuclear charge per electron decreases, causing the electron cloud to expand.
Example: The radius of a fluorine atom is $95 \ pm$, whereas the radius of a fluoride ion $(F^{-})$ is $136 \ pm$.