Explain concisely what is responsible for the variation of atomic radii in group 3A,4A,5A,6A

Shielding effect, Nuclear charge

Explanation:

moving from the left to the right of the periodic table along the same period, electrons enter into the same shells and so shielding effect is fairly constant. The number of protons, however, increase. this increase in protons lead to an increase in nuclear charge and a subsequent increase in effective nuclear charge. As the effective nuclear charge increases, the Valence electrons are more strongly pulled to the nucleus, resulting in a smaller size. anatomic radii, thus, decreases as one moves from the left to the right of the periodic table in groups 3A, 4A, 5A and 6A.

Moving down the group, atomic radii increase because the numbe of protons increase drastically and although the number of shielding electrons increase, it's not enough to hold back the nuclear charge. Effective nuclear charge, thus, increases as one moves down the groups, leading to an increase in atomic size/radius.