Question

Atomicity of Phosphorus is:

• A. 2
• B. 3
• C. 4
• D. 5

Hint:

The atomicity of an element indicates the number of atoms in one molecule of that element. For phosphorus, this is 4 in its most stable form.

Answer 1

The Correct Option is C

• Option 1: 2 - This is incorrect. Phosphorus does not exist as a diatomic molecule under normal conditions, so its atomicity is not 2.
• Option 2: 3 - This is incorrect. Phosphorus typically does not form a molecule with three atoms in its stable allotropes.
• Option 3: 4 - This is the correct answer. Phosphorus exists as P₄ molecules in its most stable allotrope (white phosphorus), which means its atomicity is 4. The atomicity of an element is the number of atoms in one molecule of that element.
• Option 4: 5 - This is incorrect. Phosphorus does not form molecules with five atoms in its most stable form. The atomicity of phosphorus is 4.

Explanation: 

Phosphorus, in its most stable allotrope, exists as a P₄ molecule, meaning it consists of four phosphorus atoms. Therefore, the atomicity of phosphorus is 4. The molecular structure of phosphorus involves a tetrahedral arrangement of phosphorus atoms in the P₄ molecule, which is the form most commonly encountered in nature.

Answer 2

Phosphorus's atomicity depends on the structure its atoms settle into when they bond together, so it helps to think about how phosphorus atoms actually bond rather than just recalling the number.

1. 2: A 2-atom molecule would need each phosphorus atom to form a very strong multiple bond with the other, similar to how nitrogen forms N2 with a triple bond. Phosphorus atoms are larger and do not form this kind of strong multiple bond with each other, so a stable P2 molecule does not exist under normal conditions.
2. 3: A 3-atom ring or chain would leave at least one phosphorus atom without enough bonds to satisfy its usual bonding capacity of three, making a 3-atom cluster too strained and incomplete to be the stable form.
3. 4: Each phosphorus atom forms three single bonds, one to each of the other three atoms in the cluster. With exactly four atoms, every atom bonds to every other atom, forming a closed, evenly strained tetrahedral shape where each atom's bonding capacity is fully and evenly satisfied. This is exactly the P4 structure that phosphorus adopts in its common white allotrope.
4. 5: A 5-atom cluster would leave the bonding arrangement uneven, some atoms would need to form four or more bonds to link every atom together, which goes beyond phosphorus's usual bonding capacity of three, so this does not match a stable structure either.

Working through how the atoms actually bond together shows that four phosphorus atoms fit together into a stable, evenly bonded tetrahedral cluster, while 2, 3, or 5 atoms cannot form a stable arrangement.

Therefore, the correct answer is 4.