How do chemist define the radius of an atom?

" atomic radius is the measure of size of an atom from its center to its boundary of outer most shell "

Explanation:

Atomic radius is the distance from the center of nucleus to the point where the electronic density is maximum.

Types of Atomic Radii  :

Atomic radii are divided into three types:

1= Metallic radius

2= Covalent radius

3= Vander Waals radius.

Metallic radius:

Metallic radius is the half of the inter nuclear distance of two adjacent ions in metallic lattice. The valance electrons are free to move in metallic lattice therefore they are weakly attracted by ions of metals.

The metallic radius is always greater than covalent radius because in in covalent bond electrons are strongly attracted between the nuclei of atoms fro example, the covalents radius of Na2 and potassium is 154 and 203 pm respectively while the metallic radius is 186 and 231 pm respectively.

Covalent Radius:

It is the one half distance between the two covalently bonded nuclei of atoms of same element in a molecule.  

Van der Waals Radius:

It is one half distance between two adjacent atoms belonging to the two neighbor molecules of element in solid state. Its magnitude is depend upon the packing of atoms in the solid state.

For example Van der Waals radius of chlorine atom is 180 pm.

Periodic trend of atomic radius:

In period:

The Van der Waals and covalent radii decreases in the period from left to right with increase of atomic number. All alkali metals in the left of periodic table have largest size.

In Group:

The atomic radii increases from top to bottom with increase of atomic number.