- The atomic number of sodium is 1 and it has 11 protons and 11 electrons with
outer electron as 3${{s}^{1}}$ whereas Na+ ion has 11 protons but only 10 electrons. - The 3s shell of ${{Na}^{+}}$ has no electron in it.
- So the outer shell configuration is 2${{s}^{2}}$2${{p}^{6}}$.
- As proton number is more than electrons, the nucleus of Na+ ion attracts outer shell electrons with strong nuclear force.
- As a result the ${{Na}^{+}}$ ion shrinks in size.
- Therefore, the size of ${{Na}^{+}}$ ions is less than Na atom.