Allotropy of phosphorus in the vapor phase: At ordinary vaporization temperatures for elemental phosphorus, the vapor primarily consists of which molecular species?

Introduction / Context:
Phosphorus exhibits multiple allotropes (white, red, black) and forms different gaseous species depending on temperature. Recognizing the dominant vapor species aids in understanding reactivity, toxicity, and physical properties in thermal processes.

Given Data / Assumptions:

• Temperatures near the vaporization of white phosphorus at ambient pressure.
• No plasma or very high-temperature dissociation conditions.
• Equilibrium among gaseous phosphorus species.

Concept / Approach:
At moderate temperatures, phosphorus vapor is dominated by P4 molecules, reflecting the tetrahedral molecular structure of white phosphorus in the condensed phase. At much higher temperatures, dissociation to P2 and other fragments becomes significant, but under typical vaporization conditions, P4 prevails.

Step-by-Step Solution:
Recall the molecular form of white phosphorus: discrete P4 tetrahedra.On vaporization near its boiling point, the gas phase retains the P4 unit predominantly.Select P4 as the correct gaseous species under ordinary conditions.

Verification / Alternative check:
Thermodynamic data and mass spectrometry indicate P4 dominance below very high temperatures where stepwise dissociation to P2 occurs.

Why Other Options Are Wrong:
P, P2, and P3 require significant bond breaking or are minor at these temperatures; they become relevant only at elevated temperatures.

Common Pitfalls:

• Confusing high-temperature dissociation products with the typical vapor species near the boiling point.
• Equating allotropes in solids with gas-phase speciation without considering temperature.

Final Answer:
P4 (tetraatomic phosphorus)