Potassium is a metallic element, but it's most commonly found as part of a salt compound, rather than as a metal. Potassium, like many other metals, is a mineral that your cells need to function properly. The potassium in your body is completely in salt form, rather than in metallic form.
Potassium is in the first column of elements on the periodic table, making it an alkali metal. This means that not only is potassium metallic in its elemental form, but it has the ability to react violently with water to produce an alkaline (basic) solution, explains Dr. Martin Silberberg in his book “Chemistry: The Molecular Nature of Matter and Change." Potassium is element number 19; it has an atomic weight of 39 atomic mass units.
The elemental form of potassium is metallic -- unreacted potassium is a conductive, silvery metal that is so soft you can cut it with a butter knife. Because potassium is extremely reactive, however, you don't find metallic potassium in nature. Instead, potassium in nature occurs in the form of the potassium cation. A cation is a positively charged particle that forms when a metal loses some of its electrons. Potassium in nature loses one electron to form a cation with a charge of +1.
Metals like potassium that form cations can't simply lose electrons -- they have to give the lost electrons to another element. When potassium loses its electron, it always gives the electron to a non-metallic element. This causes the non-metal to form a negatively charged particle, called an anion. The potassium cation is then attracted to the non-metal anion, because opposite charges attract. This results in the formation of a salt, which always consists of a cation and an anion.
In The Body and In Food
The potassium in food (bananas are rich in potassium) and in your body is in salt form. In water, the cation and anion that make up a salt separate from one another, meaning that the positively charged potassium in your body isn't stuck to a negatively charged particle; it's simply dissolved in the water-based liquid inside your cells. Potassium cation is an important mineral that has roles in nervous system function and fluid balance, explains Dr. Lauralee Sherwood in her book "Human Physiology."