Atomic radius of an element is half of the distance between the centers of two atoms of that element that are just touching each other.
- Atomic radius decreases across a period from left to right and increases down the group.
- The atoms with the largest atomic radii are located in group I and at the bottom of groups.
Moving left to right across periods, electrons are added one at a time to the outer energy shell. Electrons within a shell cannot shield each other from the attraction to protons. Since the number of protons also increases the effective nuclear charge increases across a periods. So the atomic radius increases.
Moving down the group in periodic table no. of electrons and filled electrons shells increases, but the number of valence electrons remains the same. The outer most electrons in the group are exposed to the same effective nuclear charge but electrons are found farther from the nucleus as the number of filled energy shell increases. So atomic radii increases.