Variation of Atomic Radius :
Across the period : The number of energy levels increases as you move down a group as the number of electrons increases. Each subsequent energy level is further from the nucleus than the last. Therefore, the atomic radius increases as the group and energy levels increase. As you move across a period, atomic radius decreases.

Across the first row of transition elements : From Ni to Zn there is increase in size because of the completely filled 3d and 4s orbitals. Here the nuclear charge is increased with atomic number but due to more electron -electron repulsion in 3d and 4s orbitals as compared to force of attraction between nucleus and outer orbital electrons there is little expansion in size as compared to other elements in 3d series.

Down the group : In general, atomic radius decreases across a period and increases down a group. Down a group, the number of energy levels (n) increases, so there is a greater distance between the nucleus and the outermost orbital. This results in a larger atomic radius.

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