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Which of the following is the correct formula for the compound calcium phosphate?

Ca4(PO4)3

Ca3(PO4)2

CaPO4

Ca2(PO4)3

Correct Answer

🔬 SC • Science Concept & PrincipleGeneral ScienceChemistry

Option B

Ca3(PO4)2

Quick Summary:

The chemical formula of a compound is determined by the valencies of its constituent ions using the criss-cross method. Calcium phosphate consists of calcium ions ( $Ca^{2+}$ ) and phosphate polyatomic ions ( $PO_{4}^{3-}$ ).

🔬SCScience SolutionConcept & Principle

💡 Explanation

💡 Everyday Analogy (Real-World Intuition)

Think of it like a dance-partner pairing where one calcium provides 2 hands and the phosphate needs 3; to balance the total hands (charge), 3 calcium ions (total 6 hands) must pair with 2 phosphate ions (total 6 hands).

🧠 Memory Mnemonic / Shortcut Aid

Remember 'Cross the valency, simplify the contingency' to ensure the compound is electrically neutral.

🔢 Key Formulas

$Ca^{2+} + PO_{4}^{3-} \rightarrow Ca_{3}(PO_{4})_{2}$ — Formation of calcium phosphate via charge balancing

⚙️ Working Principle

To form a neutral ionic compound, the total positive charge must equal the total negative charge. Calcium has a valency of $+2$ and the phosphate radical has a valency of $-3$ . By crossing the valencies, the $2$ subscript is assigned to the $PO_{4}$ group and the $3$ subscript to the $Ca$ atom, resulting in $Ca_{3}(PO_{4})_{2}$ .

📌 Key Points

▸
Calcium is an alkaline earth metal in Group 2, always forming a $Ca^{2+}$ ion.
▸
The phosphate ion ( $PO_{4}^{3-}$ ) is a polyatomic ion with a net charge of $-3$ .
▸
Ionic compounds must always be electrically neutral (net charge = 0).

✅ Advantages

▸
Ensures chemical neutrality in ionic bonding.

❌ Disadvantages / Limitations

▸
Does not account for transition metals with variable valencies without specifying oxidation states.

🛠️ Applications / Uses

▸
Major component of human bones and teeth (hydroxyapatite).
▸
Used as a fertilizer and food supplement.

📄 Additional Information

▸
Calcium phosphate is known for its low solubility in water, which is vital for the structural integrity of skeletal systems.
▸
Option A ( $Ca_{4}(PO_{4})_{3}$ ) is chemically incorrect because it does not balance the charges ( $+8$ vs $-9$ ). Option C ( $CaPO_{4}$ ) ignores the $+2$ and $-3$ valencies. Option D ( $Ca_{2}(PO_{4})_{3}$ ) results in a net charge of $+4 - 9 = -5$ .

📊 Diagram / Illustration

✅

B is correct — Using the criss-cross method, the $2$ from Calcium and $3$ from Phosphate swap to form $Ca_{3}(PO_{4})_{2}$ , ensuring the net charge is zero.

Core Concepts Used

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Valency and Oxidation States Ionic Bonding Polyatomic Ions Chemical Neutrality

💡 EXAM TIP

Always identify the charge of common radicals like phosphate ( $PO_{4}^{3-}$ ), sulfate ( $SO_{4}^{2-}$ ), and carbonate ( $CO_{3}^{2-}$ ) first; it makes writing formulas for almost any salt significantly easier.

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