碘是卤族化学元素，化学符号是I，原子序数是53。

碘在常温下是紫色的固体，并会释放出紫色的气体。碘会升华，但并不代表碘在常压下并没有液态，其升华的原因是其固相时蒸气压足够。碘在常压下的熔点为摄氏114度。

碘可以在氢氧化钠溶液中发生自身氧化还原反应。

3I₂ + 6NaOH → 5NaI + NaIO₃ + 3H₂O

和碘化钾进一步反应，产生KI₃。

碘是一种温和的氧化剂，可以和一些金属反应，如：

也能和一些强还原剂反应，如硫化氢或联氨：

碘和强氧化剂反应，可以被氧化，如：

氯气过量，可以进一步反应，生成三氯化碘：

Iodine is a chemical element with symbol I and atomic number 53. The name is from Greek ἰοειδής ioeidēs, meaning violet or purple, due to the color of iodine vapor.

Iodine and its compounds are primarily used in nutrition, and industrially in the production of acetic acid and certain polymers. Iodine's relatively high atomic number, low toxicity, and ease of attachment to organic compounds have made it a part of many X-ray contrast materials in modern medicine. Iodine has only one stable isotope. Iodine radioisotopes, such as I, are also used in medical applications.