磷Phosphorus

磷原子最外层有 5 个电子，3p 亚层半充满，所以第一电离能比硫高。磷可以形成三个共价键（如 PCl₃、PH₃），也能利用第三电子层扩展到五个键（如 PCl₅），这是第二周期的氮做不到的。白磷 P₄ 分子为正四面体，P–P–P 键角只有 60°，张力大，所以白磷非常活泼。

Phosphorus has five outer electrons with a half-filled 3p subshell, so its first ionization energy is higher than sulfur's. It forms three covalent bonds (PCl₃, PH₃) and can also expand to five (PCl₅) using its third shell, which nitrogen in period 2 cannot. The P₄ molecule of white phosphorus is a tetrahedron with P–P–P angles of only 60°; the strain makes white phosphorus very reactive.

1669 年德国炼金术士布兰德蒸发、灼烧大量尿液，得到一种在暗处发出绿光的物质，这是第一种有明确发现者记载的元素。名称来自希腊语 phosphoros，意为“带来光的”。白磷在暗处发光是缓慢氧化产生的化学发光。

In 1669 the German alchemist Brand evaporated and heated large quantities of urine and obtained a substance that glowed green in the dark, the first element whose discoverer is recorded. The name comes from the Greek phosphoros, light-bearer. The glow of white phosphorus is chemiluminescence from slow oxidation.

白磷是蜡状固体，熔点 44.1 °C，剧毒，在空气中约 30 °C 即可自燃，须保存在水中。红磷为聚合结构，无毒，在空气中稳定，约 240 °C 以上才燃烧，用于火柴盒的摩擦面。磷在充足空气中燃烧生成白烟 P₄O₁₀：P₄ + 5O₂ → P₄O₁₀，P₄O₁₀ 与水反应生成磷酸：P₄O₁₀ + 6H₂O → 4H₃PO₄。

White phosphorus is a waxy solid melting at 44.1 °C; it is highly toxic and ignites in air at about 30 °C, so it is stored under water. Red phosphorus is polymeric, non-toxic and stable in air, burning only above about 240 °C; it is used on the striking surface of matchboxes. Phosphorus burns in plenty of air to white fumes of P₄O₁₀: P₄ + 5O₂ → P₄O₁₀, which reacts with water to form phosphoric acid: P₄O₁₀ + 6H₂O → 4H₃PO₄.

磷的最大用途是制造磷肥（过磷酸钙、磷酸铵等），农作物生长离不开磷。磷酸用于食品添加剂和金属表面处理，磷酸盐曾大量用于洗涤剂，但会导致水体富营养化，已被限制使用。磷还用于制造火柴、阻燃剂和有机磷农药。

The largest use of phosphorus is in fertilisers (superphosphate, ammonium phosphates), as crops cannot grow without it. Phosphoric acid is a food additive and treats metal surfaces; phosphates were once widely used in detergents but cause eutrophication of waters and are now restricted. Phosphorus is also used in matches, flame retardants and organophosphorus pesticides.