定义Definition
碘是第 53 号元素，符号 I，位于第五周期第 17 族，是卤素。单质碘 I₂ 是紫黑色有金属光泽的固体，易升华为紫色蒸气。碘是人体必需的微量元素，用于合成甲状腺激素。
Iodine is element 53, symbol I, in period 5 and group 17, a halogen. The element, I₂, is a shiny grey-black solid that sublimes readily to a violet vapour. Iodine is an essential trace element in the body, needed to make thyroid hormones.
元素数据Element data
| 原子序数Atomic number | 53 |
|---|---|
| 元素符号Symbol | I |
| 相对原子质量Standard atomic weight | 126.90 |
| 电子排布Electron configuration | [Kr] 4d¹⁰ 5s² 5p⁵ |
| 族 / 周期 / 区Group / period / block | 17 / 5 / p |
| 电负性（鲍林）Electronegativity (Pauling) | 2.66 |
| 常见氧化态Common oxidation states | −1, +1, +5, +7 |
性质Properties
I₂ 分子之间只有色散力，但碘原子电子多，色散力较强，所以常温下为固体。碘在水中溶解度很小，但易溶于 KI 溶液（生成 I₃⁻）和有机溶剂，在己烷中呈紫色，在乙醇中呈棕色。碘是卤素中氧化性最弱的，I⁻ 是卤离子中还原性最强的。
I₂ molecules attract only by dispersion forces, but iodine has many electrons so these forces are strong enough to make it a solid. It is only slightly soluble in water but dissolves readily in KI solution (forming I₃⁻) and organic solvents, purple in hexane and brown in ethanol. Iodine is the weakest oxidising agent of the halogens, and I⁻ the strongest reducing agent among the halide ions.
检验与分析Tests and analysis
碘遇淀粉显深蓝色，可用于检验碘或淀粉。碘量法利用 I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻ 测定氧化剂含量，以淀粉为指示剂。卤代烃中碘代烃与硝酸银乙醇溶液反应最快，生成黄色 AgI 沉淀。
Iodine turns starch deep blue, a test for either. Iodometric titrations use I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻ with starch as indicator to measure oxidising agents. Among halogenoalkanes, iodoalkanes react fastest with ethanolic silver nitrate, giving yellow AgI.
来源与用途Sources and uses
碘最初从海藻灰中提取，现主要来自智利硝石和含碘卤水。碘酒和聚维酮碘用于消毒；食盐加碘（KIO₃ 或 KI）预防碘缺乏引起的甲状腺肿。放射性 ¹³¹I 用于甲状腺疾病的诊断和治疗。
Iodine was first obtained from seaweed ash and now comes mainly from Chilean nitrate deposits and iodine-rich brines. Tincture of iodine and povidone-iodine are disinfectants; iodised salt (KIO₃ or KI) prevents goitre caused by iodine deficiency. Radioactive ¹³¹I is used to diagnose and treat thyroid disease.
化学性质Chemical properties
与硫代硫酸根反应：I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻
With thiosulfate: I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻
遇淀粉显深蓝色
Turns starch deep blue
可被氯气从碘化物中置换出来：Cl₂ + 2KI → 2KCl + I₂
Displaced from iodides by chlorine: Cl₂ + 2KI → 2KCl + I₂
年表Timeline
- 1811
库尔图瓦在海藻灰中发现碘
Courtois discovers iodine in seaweed ash
- 1813
盖-吕萨克确认碘为元素并命名（希腊语“紫色”）
Gay-Lussac confirms it as an element and names it from the Greek for violet
- 1924
美国开始推广加碘盐
Iodised salt is introduced in the United States