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:
- [3(Ca$_3$(PO$_4$)$_2$).CaF$_2$]
- [3(Ca$_2$(PO$_4$)$_2$).Ca(OH)$_2$]
- [3(Ca$_3$(PO$_4$)$_3$.CaF$_2$]
- [3(Ca$_3$(PO$_4$)$_2$.Ca(OH)$_2$]
The Correct Option is A
Approach Solution - 1
To solve this problem, we need to identify the chemical formula of fluoroapatite, which is formed when fluoride ions (F-) convert hydroxyapatite into a harder substance. The chemical reaction replaces the hydroxyl (OH)- groups in hydroxyapatite with fluoride ions to form fluoroapatite.
Hydroxyapatite, the mineral component of tooth enamel, has the formula:
\(Ca_5(PO_4)_3(OH)\)
When fluoride ions are introduced, they replace the hydroxide ions, creating fluoroapatite:
\(Ca_5(PO_4)_3F\)
However, the formula for fluoroapatite often considered in bulk compositions is:
\(3Ca_3(PO_4)_2.CaF_2\)
Let's evaluate the given options to find the correct formula for fluoroapatite:
- \([3(Ca_3(PO_4)_2).CaF_2]\) - This formula represents fluoroapatite, where \(3(Ca_3(PO_4)_2)\) combines with \(CaF_2\).
- \([3(Ca_2(PO_4)_2).Ca(OH)_2]\) - This represents a variant of hydroxyapatite, not fluoroapatite, due to the presence of \((OH)_2\).
- \([3(Ca_3(PO_4)_3.CaF_2]\) - This formula is incorrectly structured with mismatched brackets.
- \([3(Ca_3(PO_4)_2.Ca(OH)_2]\) - This is another variant of hydroxyapatite, due to the presence of \((OH)_2\).
After analyzing the options, the correct formula for fluoroapatite is:
\([3(Ca_3(PO_4)_2).CaF_2]\)
Hence, the correct answer is the first option.
Approach Solution -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).
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