Define atomic size. How does it vary in a period and in a group ?

Part 1: Atomic size:

• Atomic size is determined by the atomic radius of an atom. Atomic radius is the distance between the center of the nucleus of an atom and its outermost shell.

• The unit of the atomic radius is Å (Angstrom).

Part 2: Trends in atomic size on moving down the group:

• On moving down the group, the number of shells increases due to which the distance between the nucleus and outermost shell also increases, therefore, the atomic size of the atoms increases.

Part 3: Trends in atomic size across the period left to right:

• On moving left to right in a period, the atomic size decreases because effective nuclear charge increases which tends to decrease the distance between the outermost shell of the atom and the nucleus.