An atomic radius decrease across Period 3 because the number of shells not increases but the number of electron increases and the atomic size decreases as the effective nuclear charge increases.
When we move in the period 3 from left to the right the atomic radius decreases because of the reason that the the number of shells don not change they will remain the same across the period but the number of the electron will increases . the effective nuclear charge will also increases. the nucleus will attract the electrons more strongly that the reason the atomic radius decreases.
Thus, number of electron increases and the number of shells will remain same , the atomic size decreases as the effective nuclear charge increases.
