F- ions convert hydroxyapatite into harder fluoroapatite. The formula of fluoroapatite is:

🧠 Substitution Without Touching the Skeleton

Hydroxyapatite is the mineral that builds tooth enamel: $[3\,\mathrm{Ca_3(PO_4)_2}\cdot\mathrm{Ca(OH)_2}]$. Fluoride does one thing — it swaps the $\mathrm{OH^-}$ pair for an $\mathrm{F^-}$ pair, leaving the calcium-phosphate skeleton untouched. So the only block that changes is $\mathrm{Ca(OH)_2 \to CaF_2}$. Everything else is identical.

🗺️ Pattern-Match Each Option

Two checks decide the answer instantly:

Check A — calcium phosphate block intact? It must read $3\,\mathrm{Ca_3(PO_4)_2}$. Any option that shows $\mathrm{Ca_2(PO_4)_2}$ or $\mathrm{Ca_3(PO_4)_3}$ is wrong by stoichiometry.

Check B — terminal block converted to $\mathrm{CaF_2}$?

| Option | Phosphate block | Terminal block | Verdict | |---|---|---|---| | (1) | $3\,\mathrm{Ca_3(PO_4)_2}$ ✓ | $\mathrm{Ca(OH)_2}$ ✗ | Still hydroxy | | (2) | $3\,\mathrm{Ca_3(PO_4)_3}$ ✗ | $\mathrm{CaF_2}$ ✓ | Wrong stoichiometry | | (3) | $3\,\mathrm{Ca_2(PO_4)_2}$ ✗ | $\mathrm{Ca(OH)_2}$ ✗ | Both wrong | | (4) | $3\,\mathrm{Ca_3(PO_4)_2}$ ✓ | $\mathrm{CaF_2}$ ✓ | Both correct |

⚡ The Recognition Move

Don't balance equations or count calciums. Just scan for the two "fingerprint" pieces: $\mathrm{Ca_3(PO_4)_2}$ and $\mathrm{CaF_2}$. Both must be present, and the phosphate must carry the prefix 3.

⚠️ Subscript Substitutions That Look Innocent

JEE option-writers love to flip a single subscript — $\mathrm{Ca_3(PO_4)_2}$ vs $\mathrm{Ca_3(PO_4)_3}$ — hoping you skim. Always read the phosphate stoichiometry character-by-character before locking in.

$\boxed{\text{Answer: (4) } [3\,\mathrm{Ca_3(PO_4)_2}\cdot\mathrm{CaF_2}]}$