### Theory:

The atomic radius is defined as the distance between the nucleus centre and the outermost shell containing the valence electron.
The radius of a separated atom is not possible to measure directly.

Except for noble gases, the atomic radius is usually referred to as a covalent radius or metallic radius depending on the bonding between the concerning atoms.

The atomic radius in metal atoms is known as metallic radius. It's defined as half the distance between adjacent metal atoms' nuclei.

Covalent radius is a term used to describe the atomic radius of nonmetallic elements. It is defined as half the distance between the adjacent nuclei of two covalently bonded atoms of the same element in a molecule.
For example, let us consider the $$H_2$$ molecule. The distance between the two hydrogen nuclei of the molecule is $$0.74$$ Å. So its covalent radius is $$0.74/2$$ = $$0.37$$ Å. When you see the variation of the atomic radii in the periodic table, there are two different trends.

From left to right, the atomic radius of the elements decreases, whereas along with the groups, from top to bottom, the atomic radius increases.
The positive charges impose a strong attraction over the electrons, and thus the electron cloud shrinks towards the nucleus, which results in a decrease in the atomic size.

The above figure shows how the atomic radius decreases from lithium to boron.