Potassium (K)
“A metal so eager to throw away its single outer electron that it dances across water in a burst of lilac flame — while your nerves and banana plants calmly wait for its ions.”
How its electrons are arranged
Computed from the aufbau principle, with known real-world exceptions applied automatically.
The notes
Characteristics
- Electron configuration [Ar] 4s¹ — a lone valence electron surrendered so readily that nature never shows free potassium: it always travels as the K⁺ ion.
- Reacts violently with cold water to give potassium hydroxide and hydrogen gas; the released heat ignites the hydrogen, and the flame burns a beautiful lilac (violet).
- The lightest metal after lithium — it floats on water (density 0.86 g/cm³) — and is stored under kerosene to keep air and moisture away.
- Here the fourth shell opens: potassium starts Period 4 because the 4s subshell fills before 3d, a straightforward consequence of the energy order inside atoms.
A silvery-white metal soft enough to slice with a butter knife — the freshly cut face gleams like polished steel, then clouds over within seconds as air attacks it. It is so light (density 0.86 g/cm³) that a lump of it floats even on water while it reacts, riding on a cushion of hydrogen bubbles.
Where you meet it
✦ Uses
- Potassium fertilisers — KCl, "muriate of potash", the K in NPK — gobbled up by the banana, sugarcane, jute and vegetable fields of Bangladesh in thousands of tonnes a year.
- Potassium hydroxide (KOH) — the electrolyte inside alkaline batteries and the alkali used to make soft liquid soaps and photographic developer.
- Potassium nitrate (KNO₃) — the saltpetre of gunpowder, plus the oxidiser in matchheads and fireworks, whose lilac flame colours festival rockets.
- Potassium chloride in oral rehydration saline (ORS) and hospital drips — replacing the K⁺ that nerves, muscles and the heartbeat run on.
◍ Everyday examples
- Bananas and coconuts are famously rich in potassium — a single medium banana packs roughly 400 mg of K⁺.
- Every sachet of ORS mixed during a bout of diarrhoea contains potassium chloride alongside common salt and sugar.
- The violet sparks in a fireworks finale and the fizz of a matchhead being struck both betray potassium’s presence.
History & name
Potash — the white salt washed out of wood and plant ashes, long stored in pots, hence "pot-ash" — had served soap-boilers and glassmakers since antiquity, and farmers knew that burning banana stems and husks yields an ash especially rich in it. In 1807 Humphry Davy passed an electric current through molten caustic potash (KOH), and shiny droplets of a brand-new metal burst from the cathode — the first metal ever isolated by electrolysis. Because it came from potash, Davy called it potassium; the symbol K honours the older Latin name kalium, from the Arabic al-qalyah, "plant ashes".