How does atomic radius change from top to bottom in a group in the periodic table? a it first increases, then decreases. b it tends to decrease. c it first decreases, then increases. d it tends to increase?

In a period (row) of the periodic table, if you move to the right, although the atomic number increases and, therefore, the number of electrons also increases, the atomic radius decreases. This is because, as the number of protons in the nucleus increases, the effective (positive) nuclear charge is increasing, and for this reason the nucleus attracts electrons (negative charges) with greater intensity, causing the element is getting smaller.

Remember that the effective nuclear charge is the force of attraction that the nucleus exerts on any of the electrons.

When you drop in a group (column) of the periodic table the effective nuclear charge remains practically constant, but the number of electrons added with respect to the previous element of the same group increases in a complete layer (which can be 8 electrons or 18 electrons depending on the element in case of having orbitals d). These new layers of electrons are further distanced from the atomic nucleus and the atomic radius is greater when you fall in a group.

In summary, the atomic radius increases from right to left in a period and from top to bottom in a group in the periodic table.