1.10 Phosphorus

Phosphorus is a heavier congener of nitrogen, and forms some analogous compounds, such as phosphine (PH₃), a toxic gas that spontaneously ignites in air, giving off ghostly flashes of light. Phosphorus' name means light bearer, and one of the forms of pure phosphorus also spontaneously burns with an otherworldly glow. Like sulfur, phosphorus cannot form diatomic molecules.

In compounds, both nitrogen and phosphorus can have a valence of either 3 or 5. Pentavalent nitrogen occurs in nitrates, such as nitric acid HNO₃, where three oxygen atoms surround an atom of nitrogen in a triangular shape, two of them double bonded.

Phosphorus has been theorized to form compounds analogous to nitrates, but these are unstable because of the larger size of the phosphorus atom. Instead, phosphates are most often based on the structure of phosphoric acid, H₃PO₄. Like sulfuric acid and the sulfate group, the phosphate group is tetrahedral.

Nitrogen and phosphorus are part of the series of elements called pnictogens, along with arsenic, antimony, and bismuth.

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