Question

What is meant by size of an atom and on what factors does it depend? Discuss its variation in the Periodic Table.

Answer 1

• The distance from the centre of nucleus to the outermost shell of electrons in an atom is known as the size of atom or atomic size. It is also denoted by the term atomic radius.

• It depends on nuclear charge and number of shells of an atom.

1. Variation of atomic size or radius in a period:- On moving from left to right in a period, number of shells remains constant but the nuclear charge increases due to increase in atomic number. As a result, the outermost shell is pulled more effectively and becomes closer to the nucleus leading to decrease in the size of atom. Hence, in general, the atomic size decreases on moving from left to right in a period.
2. Variation of atomic size or radius in a group :- On moving down a group, the nuclear charge increases with increase in atomic number, but at the same time, number of shells also increases. Thus, outermost shell (valence shell) becomes farther away from the nucleus resulting in increase of atomic size. Hence, atomic radius increase on moving down a group.