Define covalent radius.

Explanation

Covalent radius
It is defined as one half of the distance between centre of nuclei of two adjacent atoms, which are covalently bonded.
i) Atomic radius down the group
Down the group, atomic radius increases. As we move down the group, in each step number of electron energy shells increases. As a result, radius of atom increases down the group.
ii) Atomic radius along the period
Along the period, atomic radius decreases from left to right. This is because nuclear charge increases along the period. One additional electrons is added into same energy shell: As a result, effective nuclear charge increases. Consequently, electrons are attracted towards the nucleus with greater force and radius decreases regularly.