The chemical symbol for potassium is K.
The symbol K comes from kalium, the Neo-Latin name for potassium. Potassium is element 19 and belongs to the alkali metals, a group known for highly reactive metals with one outer-shell electron. In nature, potassium is found in compounds rather than as a free metal because it reacts readily with water and oxygen.
Potassium was isolated by Humphry Davy in 1807 using electrolysis of molten potassium hydroxide. The metal is soft and silvery, but it quickly tarnishes in air. Its reaction with water produces potassium hydroxide and hydrogen gas, with enough heat to ignite the hydrogen under suitable conditions.
Potassium ions are essential for nerve signalling, muscle contraction, and fluid balance in living organisms. The symbol is easy to confuse with P, which represents phosphorus, or with Po, which represents polonium.