Question

Why sodium ion is smaller then sodium atom while fluoride ion is bigger then fluorine atom

Answer 1

sodium ion is made by loosing one electron and form cation of one positive charge .

when sodium looses its electron the positive charge of nucleus per electron increases ,  there is more electrostatic force of electron  between electron and nucleus . hence size will decreases.

fluorine ion made by accepting one electron and form anion of one negative charge.

when fluorine accept one electron the positive charge of nucleus per electron decreases, there is small electrostatic force of attraction between electron and nucleus.Hence size will increases.

hope it help