Question

The 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:

• A. [3(Ca$_3$(PO$_4$)$_2$).CaF$_2$]
• B. [3(Ca$_2$(PO$_4$)$_2$).Ca(OH)$_2$]
• C. [3(Ca$_3$(PO$_4$)$_3$.CaF$_2$]
• D. [3(Ca$_3$(PO$_4$)$_2$.Ca(OH)$_2$]

Answer 1

Answer: (A)

[3(Ca$_3$(PO$_4$)$_2$).CaF$_2$]

Answer 2

Explanation:

- Hydroxyapatite, [Ca5(PO4)3(OH)], is present in tooth enamel.

- When fluoride ions (F−) are introduced, they replace the hydroxide ions (OH−), forming fluoroapatite:

Ca5(PO4)3(OH) + F− → Ca5(PO4)3F.

- Fluoroapatite has the molecular formula [3(Ca3(PO4)2·CaF2)], which is harder and more resistant to decay.

Final Answer: Option (1).