Atomic radius & ionic radius are the periodic properties which are directly or indirectly connected to the electronic configuration of the atoms & shows gradation on moving down a group or along a period.

Atomic radius is defined as the distance from the centre of the nucleus to the outer most shell containing the electrons. It evaluates the size of an atom. It is of three types:

A] Covalent radius – It is defined as the one half of the distance between the centres of the nuclei of two similar atoms bonded together by a single covalent bond.

Covalent radius = Distance between two nuclei in the bonded atoms/2

B] Metallic radium – It is defined as the one half of the distance between the centres of the nuclei of two adjacent atoms in a metallic crystal.

C] Van der Waal’s radius – It is defined as one half of the distance between the nuclei of two atoms of the same substance at their closest approach. Closest approach of two atoms means the stage when the atoms are unable to come closer without bond formation.

Ionic radii of an ion may be defined as the distance from its nucleus to the point up to which the nucleus has the influence on the electron cloud of the ion.

Ionic radius means the radius of an ion [cation or anion]. Ionic radii of an ion may be defined as the distance from its nucleus to the point up to which the nucleus has the influence on the electron cloud of the ion. The ionic radii can be determined by measuring the distances between the cations and anions in ionic crystals.

Since a cation is formed by getting a rid of electron from the atom, the cation contains lesser electrons than the parent atom, resulting in an increment in the effective nuclear charge. Thus, the size of the cation is smaller as compared to that of its parent atom. For instance, the ionic radius of sodium ion is 95 pm, whereas the atomic radius of Na atom is 186 pm.

On the other hand, in case of anion the size of the anion is larger as compared to the size of the parent atom. This is because in case of the anion the nuclear charge remains the same but more electrons get added to the atom resulting in an increased repulsion between the electrons and a reduction in the effective nuclear charge. For instance, the ionic radius of the fluoride ion is 136 pm, whereas the atomic radius of fluorine atom is 64 pm.