The radius of an atom is known as its atomic radius. It is used to determine the size of an atom. If the element is a metal, the atomic radius is the metallic radius; if it is a nonmetal, the atomic radius is the covalent radius. The internuclear distance separating the metal cores in a metallic crystal is half the metallic radius. In solid copper, for example, the internuclear distance between two neighbouring copper atoms is 256 pm. As a result, the metallic radius of copper is calculated to be,

Metallic radius of copper = 256/2=128 pm.

In a covalent molecule, the covalent radius is the distance between two atoms when they are connected by a single bond. The distance between two chlorine atoms in a chlorine molecule, for example, is 198 pm. Thus, the covalent radius of chlorine is,

Covalent radius of chlorine = 198/2 =99 pm.

The radius of an ion is referred to as its ionic radius (cation or anion). The distances between the cations and anions in ionic crystals can be used to compute the ionic radii. Because a cation is generated by removing an electron from an atom, it has fewer electrons than the parent atom and hence has a higher effective nuclear charge. As a result, a cation is smaller than its parent. The ionic radius of Na+ is 95 pm.while the atomic radius of the Na atom is 186pm. An anion, on the other hand, is larger than its parent atom. This is due to the fact that an anion has the same nuclear charge as the parent atom but contains more electrons, resulting in higher electron repulsion and a drop in the effective nuclear charge. The ionic radius of the F-  ion, for example, is 136 pm ,while the atomic radius of F the atom is