Question

why sodium ion is smaller than sodium atom while fluoride ion is bigger than fluorine atom

Answer 1

Sodium is a metal. It loses an electron when forming compounds. The loss of the outermost electron also means that all the electrons in Na^+ are more tightly held than they are in the atom (a sodium atom has 11 protons and 11 electrons, but a sodium ion has 11 protons and only 10 electrons). Thus, as expected the cation is smaller than its parent atom.

The fluoride ion has an extra electron compared to the fluorine atom, and electrons (being negative) repel each other. Since there are more electrons trying to keep away from each other, the ion is larger than the atom.