Atomic radius is the radius of an atom. It measures the size of an atom. If the element is

a metal, then the atomic radius refers to the metallic radius, and if the element is a

nonmetal, then it refers to the covalent radius. Metallic radius is calculated as half the

internuclear distance separating the metal cores in the metallic crystal. For example, the

internuclear distance between two adjacent copper atoms in solid copper is 256 pm.

Thus, the metallic radius of copper is taken as. $\frac{256}{2}pm=128pm$

Covalent radius is measured as the distance between two atoms when they are found

together by a single bond in a covalent molecule. For example, the distance between two

chlorine atoms in chlorine molecule is 198 pm. Thus, the covalent radius of chlorine is

taken as . $\frac{198}{2}pm=99 pm$

Ionic radius means the radius of an ion (cation or anion). The ionic radii can be calculated

by measuring the distances between the cations and anions in ionic crystals. Since a cation

is formed by removing an electron from an atom, the cation has fewer electrons than the

parent atom resulting in an increase in the effective nuclear charge.

Thus, a cation is smaller than the parent atom. For example, the ionic radius of ion is

95 pm, whereas the atomic radius of Na atom is 186 pm. On the other hand, an anion is

larger in size than its parent atom. This is because an anion has the same nuclear charge,

but more electrons than the parent atom resulting in an increased repulsion among the

electrons and a decrease in the effective nuclear charge. For example, the ionic radius of

F
–
ion is 136 pm, whereas the atomic radius of F atom is 64 pm.