The atomic radius is the distance between the atomic nucleus and the atom’s outermost stable electron orbital. Because rising effective nuclear force on electrons causes the atom to shrink, it tends to decrease during a period from left to right. Because of the inclusion of a new energy level, the atomic radius normally increases as one moves down a group (shell). However, because the quantity of electrons has a greater effect than the sizable nucleus, atomic radii tend to increase diagonally. Lithium, for example, has a smaller atomic radius (145 picometer) than magnesium (150 picometer).
The atomic radius can be classified in four ways:
Covalent radius is half the distance between two singly bound atoms in a diatomic molecule.
Van der Waals radius: half the distance between the nuclei of various molecules in a covalent lattice.
Metallic radius is defined as half the distance between two adjacent atom nuclei in a metallic lattice.
Ionic radius is defined as half the distance between two nuclei of an ionic compound.