Atomic Radius Within a Period

Since the number of shells increases in the same group from top to bottom (by the period number increases), the atomic radius also increases. This means that the electron cloud around the nucleus becomes larger. The increase in the number of electrons causes them occupy a new energy level and orbitals. A higher energy level is always further from nucleus. Within a period, if the number of protons and electrons increases, the nuclear attraction force increases. This attraction force prevents an enormous increase in atomic radius. Atomic Radius Within a Period 

The atomic radius usually decreases from left to right in a period. It may be thought that the atomic radius has to increase because of the increase in the number of electrons in a period (in the same shell). However, the number of protons also increases by as much as that of electrons. 

Increasing the number of protons increases the nuclear attraction force on the electrons. 

since the intensity of nuclear attraction force per one electron increases, the atomic radius decreases from left to right in a period. 

Generally the atomic radius increaes from right to left and from top to the bottom. 

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