Potassium · element 19
The metal that fizzes on water, fires your nerves, and is quietly radioactive inside you
Key facts
Drop a piece of potassium into water and it skates about, hisses and bursts into a lilac flame. Yet you need potassium every day to stay alive. An adult body holds well over 100 grams of it, and every heartbeat and every nerve impulse depends on potassium ions moving across cell membranes.
Its symbol, K, comes from kalium, the Latin word derived from the Arabic for plant ashes. The English name comes from potash, the residue left when wood ash was boiled in a pot, which was the original source of potassium compounds for making soap and glass.
Where it is found
Potassium is among the eight most abundant elements in the Earth's crust. It is part of feldspars and micas, the common minerals that make up granite and many clays. Because its compounds dissolve readily, weathering carries potassium to the sea and into evaporite deposits. Sylvite and carnallite, both potassium chlorides, are mined in Saskatchewan in Canada, in Russia and Belarus, and in Germany. Seawater holds about 0.4 grams per litre, much less than sodium, because clays and living things pull potassium out preferentially.
Living things concentrate potassium. Bananas, potatoes, beans and leafy greens are all rich in it. Inside your cells potassium is the dominant positive ion, while sodium dominates outside. Pumps in every cell membrane spend a large share of your body's energy maintaining that difference, and it is the basis of every nerve signal.
How it was discovered
Potassium was the first metal isolated by electricity. In October 1807 Humphry Davy at the Royal Institution in London passed a current from a large voltaic pile through slightly damp molten potash. Small globules of a silvery metal formed at the negative electrode and burst into flame. His cousin recorded that Davy danced around the laboratory in delight. A few days later he isolated sodium the same way. This was direct proof that the alkalis, long assumed to be elements themselves, were compounds of new metals.
What we use it for
Around nine tenths of mined potassium goes into fertiliser. Plants take up large amounts, and it is one of the three major nutrients, alongside nitrogen and phosphorus, listed as N-P-K on fertiliser bags. Potassium chloride is also sold as a salt substitute for people limiting sodium.
Potassium hydroxide makes liquid soaps and is the electrolyte in alkaline batteries. Potassium nitrate is in gunpowder and fireworks, potassium carbonate goes into glass, and potassium permanganate is a purple disinfectant and oxidiser. The metal itself is rarely used, though a sodium-potassium alloy that is liquid at room temperature has served as a coolant in some nuclear reactors.
Reading the numbers
Potassium melts at about 63 °C, cooler than a cup of tea, and boils at about 759 °C. Its density is 0.862 g/cm³, less than water, so it floats even while it reacts. It is soft enough to cut with a knife, and a fresh surface is silvery for only a few seconds before tarnishing. The configuration [Ar] 4s1 shows a single electron beyond a full argon core. Removing it costs just 419 kJ per mole, the lowest first ionisation energy of any element up to this point in the table, while the second ionisation energy leaps to over 3000 kJ per mole. That one loose electron defines potassium as a group 1 alkali metal: it forms K+ ions, reacts violently with water, and is more reactive than sodium above it because its outer electron is further from the nucleus and more weakly held. Its electronegativity of 0.82 is among the lowest of all elements.
Safety and the environment
Metallic potassium is stored under oil because it reacts with moisture and oxygen in the air, and old samples can grow an orange peroxide crust that explodes when cut. Potassium fires must never be tackled with water. In the body, potassium levels are tightly controlled by the kidneys; too little causes muscle weakness and heart rhythm problems, and too much is equally dangerous, which is why intravenous potassium is given slowly. Potassium fertiliser causes fewer environmental problems than nitrogen or phosphorus because it does not drive algal blooms.
Did you know?
- About one atom in every eight thousand of natural potassium is radioactive potassium-40, and the potassium in an adult body produces thousands of radioactive decays every second.
- Bananas are rich enough in potassium that the informal banana equivalent dose is sometimes used to put small radiation exposures into perspective.
- Potassium burns with a lilac flame, which is how chemists identify it in a flame test.
- Potassium superoxide is used in rebreathers and submarines because it absorbs carbon dioxide and releases oxygen at the same time.
- Most of the argon in the air was produced by the decay of potassium-40 in rocks over billions of years.
- Potassium was the first metal ever isolated by electrolysis, in 1807.
More in the alkali metals
These share a column with potassium, so their outer electrons are arranged in a similar way and they tend to behave alike.
See all 6 alkali metals on one page, or browse all 118 elements.
Compare Potassium with another element
Each comparison has a side-by-side data table and a plain-English summary of the differences.
Frequently asked questions
Why does potassium explode in water?
Potassium gives up its single outer electron very easily, and it reacts with water to form potassium hydroxide and hydrogen gas. The reaction releases enough heat to ignite the hydrogen, and the metal itself may be thrown apart as it reacts, producing a lilac flame and small explosions.
Why is the symbol for potassium K?
The symbol comes from kalium, the Latin name derived from an Arabic word for plant ashes, which were the original source of potassium compounds. The English name potassium comes from potash, so the two names have different roots.
Do bananas really have a lot of potassium?
Yes, a medium banana contains a few hundred milligrams, though potatoes, beans, spinach and dried fruit contain as much or more per serving. All of them are good sources, and most people get plenty from a varied diet.
Is potassium a metal?
Yes. Potassium is a soft, silvery alkali metal in group 1, so reactive that it must be stored under oil. The potassium in food and in your body is not the metal but potassium ions, which are stable and safe.
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Written by Anthony. Updated September 2026. Numerical data from Periodic-Table-JSON (CC BY-SA), cross-checked against the Potassium reference article. Spotted an error? Tell us.