Which atom is larger? Zr or Sn and why?

Answer 1

Atomic radius increases as you go from right to left across the periodic table and as you go down in a group.

Zr and Sn are both in period 5 but Zn is to the left of Sn, so it would be larger.

Explanation. Since they are both in the same period, that means they both have the same outermost orbit containing electrons. In this case, that would be n = 5 (fifth principle quantum number), so the outermost electrons are the same distance from the nucleus. BUT, Sn has more protons in the nucleus pulling those electrons closer, so it would tend to be smaller in radius. Hope this makes sense.