Question

how does atomic radius change as you move down a group/family and as you move across a period/row on the periodic table? what causes this trend?

Answer 1

When going down a group the no of orbits increases. That is n value increases (n=principle quantum number). When the number of atomic orbitals increases, the radius gradually increases. (going down the group means the number of obits that electrons can fill will increase)

And the atomic radius from left to right in the periodic table actually DECREASES. That is due to increase in electronegetivity to the right side of the periodic table.