Question

How and why does the atomic size vary as you go:
1. From left to right across a period?
2. Down a group?

Answer 1

Atomic size refers to the radius of an atom i.e. the distance between the centre of the nucleus and the outermost shell of an atom.

1. Atomic size decreases on moving from left to right across a period. This is due to the increase in nuclear charge which tends to pull the electrons closer to the nucleus and reduces the size of the atom. [1.5 marks]

2. Atomic size increases on moving down a group. This is due to the addition of new shells which increases the distance between the outermost electrons and the nucleus even though nuclear charge increases. [1.5 marks]