Allotropes of phosphorus

Elemental phosphorus can exist in several allotropes, the most common of which are white and red solids. Solid violet and black allotropes are also known. Gaseous phosphorus exists as diphosphorus and atomic phosphorus.

White phosphorus, yellow phosphorus or simply tetraphosphorus (P₄) exists as molecules made up of four atoms in a tetrahedral structure. The tetrahedral arrangement results in ring strain and instability. The molecule is described as consisting of six single P–P bonds. Two different crystalline forms are known. The α form, which is stable under standard conditions, has a body-centered cubic crystal structure. It transforms reversibly into the β form at 195.2 K. The β form is believed to have a hexagonal crystal structure.

White phosphorus is a translucent waxy solid that quickly becomes yellow when exposed to light. For this reason it is also called yellow phosphorus. It glows greenish in the dark (when exposed to oxygen), is highly flammable and pyrophoric (self-igniting) upon contact with air as well as toxic (causing severe liver damage on ingestion and phossy jaw from chronic ingestion or inhalation). The odour of combustion of this form has a characteristic garlic smell, and samples are commonly coated with white "diphosphorus pentoxide", which consists of P₄O₁₀ tetrahedral with oxygen inserted between the phosphorus atoms and at their vertices. White phosphorus is only slightly soluble in water and it can be stored under water. Indeed, white phosphorus is only safe from self-igniting when it is submerged in water. It is soluble in benzene, oils, carbon disulfide, and disulfur dichloride.

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