Animated Solution for Chemistry - s and p-Block Elements: The conversion of hydroxyapatite occurs due to presence of $F^{-}$ ions in water. The correct formula of hydroxyapatite is

Have you ever wondered what makes our teeth so incredibly strong and capable of withstanding decades of chewing and grinding? The secret lies in the outermost layer of our teeth, known as the enamel. This biological armor is not just dead tissue; it is a highly structured, crystalline mineral matrix. The primary component of this matrix is a complex mineral called hydroxyapatite.

Understanding the composition of our teeth is a classic application of s-block chemistry in biology. Calcium, an alkaline earth metal, plays a foundational role in providing structural rigidity to our skeletal system and dentition.

Let's dive into the chemical formula of this remarkable mineral. Hydroxyapatite is a complex calcium phosphate compound. Its chemical formula is written as $[3C{a}_3(P{O}_4{)}_2\cdot Ca(OH{)}_2]$.

At first glance, this formula might look intimidating, but it simply represents a repeating crystal lattice. It consists of three units of calcium phosphate, $[C{a}_3(P{O}_4{)}_2]$, associated with one unit of calcium hydroxide, $[Ca(OH{)}_2]$. The presence of the hydroxyl group ($O{H}^{-}$) is exactly what gives "hydroxyapatite" its name.

You have likely heard dentists recommend fluoride toothpaste, or you might know that many municipalities add trace amounts of fluoride to the public water supply. But why? When we consume water containing fluoride ions ($F^{-}$) or brush our teeth with fluoridated toothpaste, these ions interact directly with the hydroxyapatite crystals on the surface of our enamel.

Here is where a fascinating chemical substitution takes place. The fluoride ions, being similar in size and charge to the hydroxide ions, can slip into the crystal lattice and replace the $O{H}^{-}$ groups. The chemical reaction can be represented as:

This substitution converts the hydroxyapatite into a new mineral called fluoroapatite, which has the formula $[3C{a}_3(P{O}_4{)}_2\cdot Ca{F}_2]$.

Why does this substitution matter? Fluoroapatite is significantly harder and much more resistant to the acidic byproducts of bacteria in our mouths compared to hydroxyapatite. By replacing the hydroxide with fluoride, we effectively upgrade the armor of our teeth, preventing cavities and decay.

Returning to the core question, we were asked to identify the correct formula for the original mineral, hydroxyapatite, before any conversion takes place. Based on our chemical blueprint, the correct formula is undoubtedly $[3C{a}_3(P{O}_4{)}_2\cdot Ca(OH{)}_2]$.