Atomic radius difference

Question

Why is the atomic radii of calcium bigger than selenium?

Atomic radius / pm

Calcium, Ca 197

Selenium, Se 116

Answer 1

Think of Francium, this element as the king of large atomic radius. When doing problems like this and follow trend. Elements that are close to it have large atomic radius. This can also happen with electronegativity. Think of Florine as queen of electronegativity. Elements close to her, are most electronegative. Noble gases don't really like to "mingle" with anyone as it pertains to many ionic and covalent bonding.

Answer 2

The trend, especially in the A elements, is that radius of atoms decreases from IA to VIII in a row. This is a combination of the nuclear charge increasing (bringing electrons in) and the orbitals filling so that the electrons are not shielding each other from the nucleus. There is a huge jump in radii going from Noble Gas to IA (next row) because a new n level is required and that electron is much farther out.