The measure of the size of the atom of a chemical element is known as the atomic radius of that chemical element. Usually atomic radius is the mean distance from middle of the nucleus of an atom to the shell’s boundary that surrounds the electrons. There are various definitions of atomic radius which are non-equivalent because the physical entity of the boundary is not defined well. Atomic radius have three definitions that are widely used which are ionic radius, Van der Waals radius and the covalent radius. According to the definition, atomic radius can be applied to only isolated atoms or to atoms which are in covalently bond or condensed matter in molecules, or in excited state and ionized. According to the definition to obtain the value of atomic radius some theoretical models are to be computed or some experimental measurements are to be conducted.

A gradual decrease in the atomic size of the elements is observed from left to right through a period because all the electrons adds to the same shell within a period of elements. Accordingly the protons are also been added into nucleus, thus making the atom more positively charge as the increment in proton is greater than that of electron hence, nuclear attraction increase. This indicates that electron are attracted more strongly to nucleus and the atom’s shell is pulled closer thus resulting in the decrease of atomic radius. Atomic radius is seen to increase when traversed down in group as the quantum number increases the valence electrons attains higher level. Thus making the valence electrons loosely held as they get away from nucleus and thus result in the increase of atomic radius.