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

Step 1: Understanding the Question.
We need to recall how many phosphorus atoms combine to form one molecule of phosphorus in its ordinary, stable form.

Step 2: Key Idea.
Different elements settle into different fixed atomicities based on their common allotrope: oxygen is O2 (atomicity 2), ozone is O3 (atomicity 3), and sulfur is S8 (atomicity 8). Phosphorus follows this same pattern of forming a fixed multi-atom cluster rather than existing as single atoms or simple pairs.

Step 3: Applying it to phosphorus.
The common, stable form of phosphorus is white phosphorus, which exists as a P4 molecule, four phosphorus atoms arranged in a tetrahedron.

Step 4: Final Answer.
The atomicity of phosphorus is 4.

Answer 2

Looking at where phosphorus sits in the periodic table gives another way to check its atomicity, since elements in the same group often show related bonding behavior.

1. 2: An atomicity of 2 belongs to elements like nitrogen, which sits directly above phosphorus in group 15. Nitrogen can form a strong triple bond to make N2, but phosphorus atoms are too large to form an equally strong multiple bond with each other, so phosphorus does not follow nitrogen's 2-atom pattern.
2. 3: There is no common, stable allotrope of any group 15 element built from just three atoms, so this does not match phosphorus's known chemistry.
3. 4: Phosphorus, being a larger atom than nitrogen, instead forms single bonds to three separate neighboring atoms, and the most stable way to arrange four atoms so that each one bonds singly to every other atom is a tetrahedron. This is exactly the P4 structure seen in white phosphorus, phosphorus's most common and stable form.
4. 5: No common allotrope of phosphorus is built from five atoms in a stable cluster, this number does not correspond to any known, stable form of the element.

Since phosphorus cannot follow nitrogen's 2-atom pattern and no stable 3-atom or 5-atom cluster exists for it, the tetrahedral 4-atom P4 structure is phosphorus's actual stable form.

Therefore, the correct answer is 4.