The \( \mathrm{F^-} \) ions make the enamel on teeth much harder by converting hydroxyapatite (the enamel on the surface of teeth) into much harder fluoroapatite having the formula.
\( \left[ 3(\mathrm{Ca_3(PO_4)_3}) \cdot \mathrm{CaF_2} \right] \)
\( \left[ 3(\mathrm{Ca_2(PO_4)_2}) \cdot \mathrm{Ca(OH)_2} \right] \)
\( \left[ 3(\mathrm{Ca_3(PO_4)_2}) \cdot \mathrm{CaF_2} \right] \)
\( \left[ 3(\mathrm{Ca_3(PO_4)_2}) \cdot \mathrm{Ca(OH)_2} \right] \)