a
The \({F}^{\Theta}\) ions make the enamel on teeth much harder by converting hydroxyapatite, \(\left[3\left({Ca}_{3}({PO})_{2}\right] \cdot {Ca}({OH})_{2}\right]\) the enamel on the surface of the teeth into much harder fluroappatite. \(\left[3 {Ca}_{3}\left({PO}_{4}\right)_{2} \cdot {CaF}_{2}\right]\)