Potassium readily loses the single electron in its outer valence, causing it to react very energetically to substances that strongly attract electrons. Potassium is a silver colored metal that tarnishes rapidly when exposed to air because of the oxygen and water vapor.
When exposed to air, potassium turns into potassium oxide, and under certain conditions, burns forming potassium superoxide. It has extremely energetic reactions with water, forming potassium hydroxide and hydrogen gas, which burns with a lilac-colored flame.
Potassium reacts very strongly with halogen gases and acids, forming potassium salts and, in the case of acids, helium gas. It reacts readily with a large majority non-metallic elements, including common elements such as sulfur, phosphorus and nitrogen. These reactions are often extremely energetic or even explosive. In order to be safely stored, potassium must be immersed in a highly inert substance like oil and kept in a suitable container.
Potassium is an important element for most organisms and forms a very wide variety of common compounds. It is also one of the more common trace elements in the Earth's crust. It is crucial in intracellular mechanisms, where it works opposite sodium to control the movement of water across membranes. Several edible plants have high levels of potassium compounds, including bananas and spinach.