Question

define atomic size how does it vary from period and Groups

Answer 1

Atomic size decreases across a Period from left to right as we face the Table, but INCREASES down aGroup, a column of the Periodic Table.
Atomic radius of the elements generally decreases from left to the right in a period because on moving from left to right in aperiod the nuclear charge gradually increases by one unit & one electron is also added in the electron shell.Due to  this the electrons get attracted more & more towards the nucleus.consequently the atomic radii decreases .

Atomic radius of the elements increases as we move downwords in a group because on moving down a group there is an increase in principal quantum number & thus, increase in the number of electrons shells.Therefore the atomic size is expected to increase.

Answer 2

The atomic radius of a chemical element is a measure of the size of its atoms, usually the mean or typical distance from the center of the nucleus to the boundary of the surrounding cloud of electrons.
It varies both along the period and along the group.Along the period it decreases due to increase in the number of electrons in the outermost shell which increases the intermolecular attraction and decreases its size.
Along a group it increases due to addition of new shells which increases the distance between nucleus and valence shell.