Atomic Radius of elements change in Periodic Table: Atomic size is a periodic property. In any period as one goes from left to right (i.e., as atomic number increases) the atomic radius of the elements decrease. The reason behind this is that with the increase of atomic number, the number of electrons increases, but there is no increase of electron orbit. As atomic number increases, the number of protons in the nucleus also increases. Due to higher positive charge on the nucleus it attracts the electrons more strongly. As the result the atomic radius decreases.
On the other hand when one goes from top to bottom (i.e. as atomic number increases) in a group, new electron orbits are added, hence the atomic radius increases.