Which one of the following elemental forms is not present in the enamel of the teeth?

We need to identify which elemental form is NOT present in the enamel of teeth.

Composition of tooth enamel:

Tooth enamel is primarily composed of hydroxyapatite, which has the formula:

$$Ca_5(PO_4)_3OH$$ or equivalently $$Ca_{10}(PO_4)_6(OH)_2$$

In fluoridated enamel, some hydroxyl groups are replaced by fluoride ions, forming fluorapatite:

$$Ca_5(PO_4)_3F$$

Analyzing each option:

Option A: $$Ca^{2+}$$ - Calcium is present as $$Ca^{2+}$$ ions in hydroxyapatite. This IS present in enamel.

Option B: $$P^{3+}$$ - In the phosphate group $$PO_4^{3-}$$, phosphorus is in the +5 oxidation state, not +3. $$P^{3+}$$ is NOT present in tooth enamel.

Option C: $$F^{-}$$ - Fluoride ions are present in fluorapatite, which strengthens the enamel. This IS present in enamel.

Option D: $$P^{5+}$$ - Phosphorus exists in the +5 oxidation state in the phosphate ion $$PO_4^{3-}$$. This IS present in enamel.

The correct answer is Option B: $$P^{3+}$$.