Question

why does the atomic radius change down a group/family?
number of protons (effective nuclear charge) increases as energy level increases, decreasing the radius of an atom.
number of protons (effective nuclear charge) decreases as energy level increases, decreasing the radius of an atom.
number of layers (shielding effect) increases down a group, therefore making radius bigger
number of layers (shielding effect) decreases down a group, therefore making radius bigger

Explanation:

As you move down a group in the periodic table, new energy levels (shells) are added. Each new energy level is further from the nucleus, increasing the shielding effect of inner - shell electrons. This reduces the effective nuclear charge experienced by outer - shell electrons, allowing the outer electrons to be further from the nucleus and thus increasing the atomic radius.

Answer:

Number of layers (Shielding effect) increases down a group, therefore making radius bigger