镁Magnesium

镁原子最外层有两个 3s 电子，失去后形成 Mg²⁺。Mg²⁺ 半径小、电荷高，与 O²⁻ 之间的离子键很强，MgO 熔点约 2850 °C，用作耐火材料。与钠相比，镁每个原子提供两个离域电子，金属键更强，熔点（650 °C）也高得多。

Magnesium has two 3s outer electrons and loses both to form Mg²⁺. The small, doubly charged Mg²⁺ forms strong ionic bonds with O²⁻, so MgO melts at about 2850 °C and is used as a refractory. Compared with sodium, each magnesium atom provides two delocalised electrons, so the metallic bonding is stronger and the melting point (650 °C) much higher.

1755 年布莱克区分了苦土（MgO）与石灰（CaO）。1808 年戴维电解氧化镁与氧化汞的混合物得到镁汞齐，首次分离出镁；1831 年比西用钾还原氯化镁制得较纯的镁。名称来自希腊的 Magnesia 地区。

In 1755 Black distinguished magnesia (MgO) from lime (CaO). In 1808 Davy electrolysed a mixture of magnesia and mercury oxide to obtain a magnesium amalgam, first isolating the element; in 1831 Bussy made fairly pure magnesium by reducing magnesium chloride with potassium. The name comes from the Magnesia district of Greece.

镁在空气中表面形成氧化膜而较稳定。点燃后剧烈燃烧，发出耀眼白光：2Mg + O₂ → 2MgO，也能在 CO₂ 中燃烧：2Mg + CO₂ → 2MgO + C，所以镁着火不能用 CO₂ 灭火器扑灭。镁与冷水反应很慢，与水蒸气反应较快：Mg + H₂O(g) → MgO + H₂。镁与稀酸迅速反应放出氢气。

Magnesium is protected in air by an oxide film. Once lit it burns fiercely with a brilliant white light, 2Mg + O₂ → 2MgO, and it even burns in carbon dioxide, 2Mg + CO₂ → 2MgO + C, so a CO₂ extinguisher cannot put out a magnesium fire. It reacts very slowly with cold water but faster with steam, Mg + H₂O(g) → MgO + H₂, and rapidly with dilute acids to give hydrogen.

工业上从海水或卤水中沉淀出 Mg(OH)₂，转化为 MgCl₂ 后电解熔融氯化镁制镁；也可用硅铁在高温真空中还原煅烧白云石（皮江法）。镁密度只有 1.74 g cm⁻³，镁合金用于汽车、飞机和电子产品外壳。镁还用于照明弹、烟花和格氏试剂的制备。镁离子是叶绿素的中心离子，也是人体必需的矿物质。

Industrially, Mg(OH)₂ is precipitated from seawater or brine, converted to MgCl₂, and the molten chloride electrolysed; alternatively calcined dolomite is reduced with ferrosilicon under vacuum at high temperature (the Pidgeon process). With a density of only 1.74 g cm⁻³, magnesium alloys are used in cars, aircraft and electronics casings. Magnesium is also used in flares, fireworks and the preparation of Grignard reagents. Mg²⁺ sits at the centre of chlorophyll and is an essential mineral for humans.