In the different allotropic forms of phosphorus, which molecular form corresponds to white phosphorus that is highly reactive and toxic?

Introduction / Context:
Phosphorus exists in several allotropic forms, with white (or yellow), red, and black phosphorus being the most commonly discussed in general chemistry. These allotropes differ in structure, reactivity, and physical properties, even though they are all made from the same element. White phosphorus is notorious for its high reactivity and toxicity and is often represented by a specific molecular formula in which phosphorus atoms form a particular cluster. This question tests your recall of that molecular form.

Given Data / Assumptions:

• The element in focus is phosphorus, which has multiple allotropes.
• The options list P1, P6, P4, and P5 as possible molecular forms.
• White phosphorus is known to occur as discrete molecular units in the solid state.
• We assume standard textbook descriptions of white phosphorus.

Concept / Approach:
White phosphorus consists of P4 molecules. Each P4 molecule has four phosphorus atoms arranged in a tetrahedral structure with P–P single bonds. This structure is strained and makes white phosphorus very reactive, especially in air, where it can ignite spontaneously. Red and black phosphorus have more extended polymeric structures and are not simply described as P4 molecules. Therefore, among the given options, P4 is the correct molecular form associated with white phosphorus.

Step-by-Step Solution:
Step 1: Recall the allotropes of phosphorus. White phosphorus, red phosphorus, and black phosphorus are the main forms. Step 2: Identify the structure of white phosphorus. White phosphorus is made up of P4 molecules where four phosphorus atoms form a tetrahedral cluster. Step 3: Match the molecular formula to the options. The formula P4 is explicitly given as one of the options. Step 4: Confirm that P1, P5, and P6 are not standard formulas used to describe white phosphorus.

Verification / Alternative check:
Standard inorganic chemistry textbooks and exam guides describe white phosphorus as consisting of discrete P4 molecules, each with a tetrahedral shape. They often contrast this with red phosphorus, which is described as a polymeric form where P4 units are linked, and black phosphorus, which has a layered structure. No reference is made to simple molecular species labelled P1, P5, or P6 for white phosphorus. This consistent description confirms that P4 is the only correct formula for white phosphorus among the options given.

Why Other Options Are Wrong:
Option A (P1): A single phosphorus atom is not how white phosphorus is represented in the solid state; it is not a recognised formula for this allotrope. Option B (P6): P6 is not the standard molecular unit for white phosphorus; there is no common white phosphorus allotrope described as P6 in basic chemistry. Option D (P5): Similarly, P5 is not used to represent white phosphorus in textbooks and is not associated with its known tetrahedral structure.

Common Pitfalls:
Students sometimes confuse the word "white phosphorus" with simple atomic forms or assume that a larger cluster like P6 might be more stable. Others may not clearly remember the tetrahedral P4 structure and guess randomly among the options. To avoid confusion, remember that white phosphorus is the classic example of a molecular solid made from P4 units, which are responsible for its unusual reactivity and need for storage under water to prevent ignition in air.

Final Answer:
White phosphorus corresponds to the molecular form P4.