钾（拉丁语：Kalium，化学符号：K）是原子序数为19的化学元素，银白色有光泽的1A族碱金属元素，质软，和钠的化学性质相似但更活泼。

钾的熔点、硬度低，比钠更活泼，在空气中很快氧化。钾的密度小于水，大于煤油。钾和水会产生剧烈反应（产生高温使自己熔成一个银白色的球，释放大量氢，使金属球在水面高速移动，氢气燃烧，可以看到紫蓝色的火焰）生成氢氧化钾。钾可以和卤族、氧族元素反应，还可以使其他金属的盐类还原（熔融状态下），对有机物有很强的还原作用。 容易与氧反应 在表面形成紫色的氧化钾

钾为爆炸性和易燃的物质，一般以汽油或煤油封存。

Potassium is a chemical element with symbol K (derived from Neo-Latin, kalium) and atomic number 19. It was first isolated from potash, the ashes of plants, from which its name is derived. In the Periodic table, potassium is one of seven elements in column (group) 1 (alkali metals). All elements in Group 1 have a single valence electron in the outer electron shell, which is easily removed to create an ion with a positive charge - a cation, which combines with anions to form salts. Potassium in nature occurs only in ionic salts. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and reacts vigorously with water, generating sufficient heat to ignite hydrogen emitted in the reaction and burning with a lilac-colored flame. It is found dissolved in sea water (which is 0.04% potassium by weight), and is part of many minerals.