Human teeth (and bones) are primarily composed of the mineral calcium hydroxyapatite
Calcium hydroxyapatite has the chemical formula: Ca5[(PO4)3OH]
One of the main components of this mineral is the hydroxyl ion (an ion of oxygen and hydrogen with a charge of –1
Fluoride (also an ion with a charge of –1) substitutes for the hydroxyl ion, producing calcium fluoroapatite, with the chemical formula:
Ca5[(PO4)3F]