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

i.                     Atomic size is determined by using atomic radius.

ii.                    For an isolated atom, its atomic radius is the distance between the centre of the atom (i.e. centre of its nucleus) and its outermost shell.

iii.                  In a period, atomic radius generally decreases from left to right. This is because the electrons are added to same shell and experience greater pull from the nucleus.

iv.                  Atomic radius increases in a group from top to bottom as new shells are added bringing outermost electrons farther from the nucleus.