In terms of atomic structure, explain why the atomic radius of K is larger than that of Na

Sodium and potassium are present in group 1A. Both are alkali metals.

The atomic number of sodium is 11 and potassium is 19. Sodium is present in period 3rd while potassium is present in period 4th.

Electronic configuration of sodium:

Na₁₁ = 1s² 2s² 2p⁶ 3s¹

K₁₉ = 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹

It can be seen in electronic configuration that in case of sodium the valance electron is present in third energy level while in case of K outer most electron is present in 4th energy level. So atomic structure of sodium has three energy level and K has four energy level that's why radius of potassium is greater than sodium because its the distance from center to outer most electronic shell.

Moreover as we move down the group atomic radii increased with increase of atomic number. The addition of electron in next level cause the atomic radii to increased. The hold of nucleus on valance shell become weaker because of shielding of electrons thus size of atom increased.