What is a periodic property? How the following properties vary in a group and in a period? Expain
(a) Atomic radius.

Explanation

The repetition of similar chemical properties of elements at regular intervals with increasing atomic number is called periodic property or periodicity.
(a) Atomic radius: In a period, as we go from left to right, the atomic radius gradually decreases.
Reason: As we go from left to right in a period, the differentiating electron enters into the same orbit but at the same time, the nuclear charge increases. As a result of which the effective nuclear charge over the outermost electrons increase, leading to a decrease in the atomic radius.
As we go from top to bottom in a group, the atomic raduis gradually increases.
Reason: As we go from top to bottom in a group, the valence electrons enter into new shells, even though the nuclear charge increases. As a result of which, the effective nuclear charge over the outermost electrons decrease, leading to n increase in the atomic radius.