Why are P2 molecules of phosphorus stable only at very high temperatures?

Phosphorus molecules with the formula $P_2$ are stable only at very high temperatures—typically above 1,200 °C (2,200 °F)—because of the fundamental energetic differences between phosphorus and nitrogen.

While $P_2$ is structurally analogous to $N_2$ and features a triple bond, it is not the most stable configuration for phosphorus at lower temperatures. In phosphorus, the formation of three single bonds is energetically favored over the formation of one triple bond. Consequently, as the temperature drops, these triply bonded $P_2$ molecules become unstable and condense to form tetrahedral $P_4$ molecules, in which each phosphorus atom is joined to three others by single bonds.1