铁Iron

铁失去 4s 电子生成 Fe²⁺（3d⁶），再失去一个 3d 电子生成半充满的 Fe³⁺（3d⁵），后者较稳定。⁵⁶Fe 的平均核子结合能接近最大值，恒星核聚变到铁附近即停止释放能量，因此铁在宇宙中丰度很高。

Iron loses its 4s electrons to give Fe²⁺ (3d⁶) and one more to give the half-filled, more stable Fe³⁺ (3d⁵). ⁵⁶Fe has close to the highest binding energy per nucleon, so stellar fusion stops releasing energy near iron, which explains its high cosmic abundance.

最早的铁器来自陨铁。约公元前 1200 年起，安纳托利亚和近东进入铁器时代。中国在春秋时期已掌握生铁冶炼和铸铁技术，比欧洲早约一千五百年。1709 年达比用焦炭炼铁，1856 年贝塞麦发明转炉炼钢，使钢铁大规模生产成为可能。

The earliest iron objects were made from meteoritic iron. From about 1200 BCE Anatolia and the Near East entered the Iron Age. China mastered pig-iron smelting and casting in the Spring and Autumn period, some 1,500 years before Europe. Darby smelted iron with coke in 1709, and Bessemer's converter of 1856 made mass steel production possible.

纯铁是银白色、有延展性的金属，具铁磁性，居里温度 770 °C。常温下为体心立方的 α-Fe，912 °C 以上转变为面心立方的 γ-Fe，1394 °C 以上为体心立方的 δ-Fe。铁在潮湿空气中生锈（Fe₂O₃·xH₂O），在冷的浓硫酸、浓硝酸中钝化。

Pure iron is a silvery, ductile, ferromagnetic metal with a Curie temperature of 770 °C. At room temperature it is body-centred cubic α-Fe; above 912 °C it becomes face-centred cubic γ-Fe, and above 1394 °C body-centred δ-Fe. It rusts in moist air (Fe₂O₃·xH₂O) and is passivated by cold concentrated sulfuric and nitric acids.

铁主要以钢（含碳 0.02–2.1%）和铸铁的形式使用，全球年产粗钢约 19 亿吨。铁也是哈伯法合成氨的催化剂主体。人体内约 4 g 铁，大部分在血红蛋白和肌红蛋白中负责输送和储存氧，缺铁会导致贫血。

Iron is used mainly as steel (0.02–2.1% carbon) and cast iron; world crude-steel output is about 1.9 billion tonnes a year. It is also the main component of the Haber-process catalyst. The human body holds about 4 g of iron, mostly in haemoglobin and myoglobin, which carry and store oxygen; iron deficiency causes anaemia.