Ionic Radii

An ionic radius is the radius of a cation or an anion defined by the distance between ions in an ionic compound. Cations are smaller than the parent atom, whereas anions are larger. Similar to atomic radii, ionic radii are determined by the number of electrons, the orbitals holding its valence electrons, and the nuclear charge. 

Consider lithium, which has an electron configuration of a helium core and one outermost 2s electron. The 2s electron is shielded from the nuclear charge by two 1s electrons and contributes to the atomic radius of 152 picometers.

The loss of the outermost 2s electron generates a lithium cation, which has fewer electrons but the same number of protons as the parent atom. The two 1s  electrons are held closer to the nucleus because they experience a greater effective nuclear charge than the 2s electron did. Thus, the ionic radius of the lithium cation is 60 picometers, which is much smaller than the parent atom. 

This trend is generally observed with all metal cations and their parent atoms.

In contrast, anions are larger than their parent atoms. When a fluorine atom accepts an electron, there is an additional outermost electron, but the number of protons, an