Atomic number 19

Potassium

K
+ phase
− phase

Shells Orbitals 4s

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19 electrons in 4 shells: 2, 8, 8, 1.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Alkali metal
Period: 4
Group: 1
Block: s
Standard atomic weight: 39.098(1) Da
Electron configuration: [Ar] 4s¹
Phase at 25 °C: solid
Melting point: 63.38 °C
Boiling point: 759 °C
Density: 0.89 g/cm³
Electronegativity (Pauling): 0.82
Discovered: 1807, Humphry Davy

Neighbours in the table

Ar Ca

A potassium atom’s nineteenth electron could have gone into 3d or 4s, and it went into 4s. The 4s orbital, OpenStax says, has three radial nodes, spherical surfaces where the electron is never found, and reaches closer to the nucleus. A 3d electron would be less penetrating, more shielded from the nucleus and so higher in energy. The result is [Ar] 4s¹, with 2, 8, 8 and 1 electrons in four shells, and potassium’s chemistry follows lithium’s and sodium’s.

Pulling that electron off a potassium atom costs 4.341 eV, its first ionisation energy. A second, converted from OpenStax’s table, costs 31.630 eV: it comes from the argon core, whose electrons are harder to remove. Potassium therefore stops at K⁺, an ion with argon’s configuration. Down group 1 that energy falls and reactivity rises, so potassium reacts more readily than sodium.

Los Alamos describes the metal as soft, easily cut with a knife, silvery on a fresh surface and, after lithium, the lightest known. Its density is 0.89 g/cm³, below water’s, and it melts at 63.38 °C. Moisture and oxygen both attack it, so laboratory potassium comes as sticks or beads under mineral oil or in sealed containers. Given excess oxygen it forms the superoxide KO₂, a salt of the O₂⁻ ion, not the oxide. It colours a flame pale violet.

Where it is found

No potassium metal occurs in nature; every natural atom is a K⁺ ion, in salt beds, silicates or the sea. PubChem’s element record gives a crustal abundance of 2.09 × 10⁴ milligrams per kilogram, about 2.1 per cent of the crust. Jefferson Lab, quoted in that record, ranks it eighth among the elements on Earth. Seawater holds far less, 399 milligrams per litre, small beside sodium’s.

Most potassium minerals are insoluble, and Los Alamos notes that getting the metal out of them is very difficult. The deposits worth working are salts left where ancient lakes and seas dried out. Sylvite is potassium chloride; carnallite, langbeinite and polyhalite are mixed salts carrying magnesium, and polyhalite calcium too. Mined, they are sold as potash.

The US Geological Survey puts 2024 world potash production at about 48 million metric tons of potassium oxide equivalent, 15 million of them from Canada, the leading exporter. Israel and Jordan draw theirs from the Dead Sea, which holds nearly 2 billion tons of potassium chloride. Most US output comes from two underground mines and a deep-well solution mine in southeastern New Mexico.

Why it matters

Humphry Davy made the first potassium in 1807 by passing a current through caustic potash, potassium hydroxide; Los Alamos calls it the first metal isolated by electrolysis. A few months later the same method gave Davy sodium. Group 1 ions resist reduction, the return of an electron, so the textbook route is electrolysis. Industry prefers a cheaper route: sodium metal reduces molten potassium chloride. Potassium vaporises more readily than sodium, so it distils out of the melt, and removing the vapour pushes the equilibrium towards more metal.

Potash, the US Geological Survey’s name for water-soluble potassium salts, went about 85 per cent to fertiliser in US sales in 2024. The survey lists no substitute for potassium as a plant nutrient. The rest went to industry: potassium hydroxide makes soaps and drain cleaners and is often the alkaline battery’s electrolyte, its conducting solution.

In the body, potassium is the main positive ion inside cells. A pump in the cell membrane moves sodium out and potassium in, each towards where it is already concentrated. Working against those concentration gradients is why the pump needs energy. Potassium helps set the resting potential of nerve and muscle cells, −70 mV. When a nerve fires, potassium ions leaving the cell carry positive charge out and bring the membrane back to rest, a step called repolarisation.

Properties

Properties of potassium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	19
Standard atomic weight	39.098(1) Da	Da	IUPAC/CIAAW 2024
Melting point	63.38 °C	°C	PubChem: 336.53 K
Boiling point	759 °C	°C	Los Alamos: 759 °C
Density	0.89 g/cm³	g/cm³
Electronegativity (Pauling)	0.82
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹
Phase at 25 °C	solid

Safety

Potassium metal, the uncombined element, is hazardous: PubChem’s classification gives it the signal word Danger. Water sets free a flammable gas that may ignite by itself, and contact with the metal burns the skin and eyes. It burns in moist air at room temperature, and metal that has oxidised during storage under oil is a recognised laboratory hazard in its own right. None of this touches the potassium ion in food, fertiliser or salt substitute; anyone handling the metal should read the PubChem Laboratory Chemical Safety Summary.

Questions

Why is the symbol for potassium K?

K stands for kalium, the element's Latin name, which the CIAAW history in PubChem's record traces through the Arabic qali, meaning alkali. The English name has a separate origin, potash or pot ashes, after the caustic potash, potassium hydroxide, in which the element was found. The symbol therefore follows the Latin and the name follows the English.

Why does potassium catch fire in water?

Potassium's lone outer electron is the easiest to remove of any element in its period, and ionisation energy falls further down the group, which is why potassium reacts more vigorously than sodium. Water strips that electron away, leaving hydrogen gas and a solution of potassium hydroxide. Jefferson Lab's text in the PubChem record supplies the decisive step: the reaction releases enough heat to ignite the hydrogen. Los Alamos says simply that it catches fire spontaneously on water.

Is potassium radioactive?

One of potassium's three isotopes, potassium-40, is radioactive: 0.0117 per cent of the atoms, half-life 1.248 billion years, and Los Alamos calls the radioactivity no appreciable hazard. About 89 per cent of its decays make calcium-40 by beta emission; the rest, 11 per cent, make argon-40 by electron capture, the nucleus absorbing an atomic electron. Geologists date lava by the smaller branch: once the rock has crystallised, argon can no longer escape, and its amount records the age.

What is the difference between potash and potassium?

Potassium is the element. Potash is the name for its water-soluble salts, mined or manufactured, as the US Geological Survey defines it, and in farming it means the potassium fertilisers. Muriate of potash is potassium chloride of at least 95 per cent purity mixed with sodium chloride. Sulfate of potash and langbeinite, a potassium magnesium sulfate, go to crops that cannot tolerate chloride, and about 70 per cent of US output in 2024 was those two forms.

Which foods are high in potassium?

The nutrition chapter of OpenStax Anatomy and Physiology lists meats, some fish, fruits, vegetables, legumes and dairy products as the sources, and reminds readers that the body cannot make minerals, so all of them come from the diet. Potassium is one of the six minerals needed in moderate amounts, alongside sodium, calcium, phosphorus, magnesium and chloride. The table gives its job as nerve and muscle function, and the shortage called hypokalaemia shows as weakness, fatigue and muscle cramps.

Compounds containing potassium

KCl Potassium chloride 2 atoms in one formula unit of an ionic lattice.
KI Potassium iodide 2 atoms in one formula unit of an ionic lattice.
KOH Potassium hydroxide 3 atoms in one formula unit of an ionic lattice.
KNO₃ Potassium nitrate 5 atoms in one formula unit of an ionic lattice.
KMnO₄ Potassium permanganate 6 atoms in one formula unit of an ionic lattice.