Atomic radius  & ionic radius are the periodic properties which are directly or indirectly related to the electronic configuration of their 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 measures the size of an atom.It is of 3 types:

A)    Covalent radius- It is the one half of the distance between the centres of the nuclei of two adjacent similar atoms joined to each other by single covalent bond.

Covalent radius = inter nuclear distance in the bonded atoms/ 2

B)    Metallic radius- It is defined as half the distance between the centres of the nuclei of two adjacent atoms in the metallic crystal

C)    Van der waal’s radius- It is defined as one half of the inter nuclear distance between 2 similar adjacent atoms belonging to the two neighbouring molecules of the same substance in the solid state.

Ionic radius means the radius of an ion (cation or anion). It is defined as the distance from the centre of the nucleus of the ion upto which it exerts its influence on the electron cloud.The ionic radii can be calculated by measuring the distances between the cations and anions in ionic crystals.