← Periodic table · Alkali metals
Potassium (K)
Potassium (symbol K) is the chemical element with atomic number 19: an alkali metal, solid at 0 °C, with an atomic mass of 39.098 u. Discovered in 1807 (Humphry Davy).
Potassium is an alkali metal even more reactive than sodium: it bursts into flame on contact with water. In nature it is abundant as salts, mined as fertilizer (potash). In our cells, it works in tandem with sodium to carry nerve impulses and keep the heart beating. One of its isotopes is naturally radioactive, which makes each of us a faint source of radioactivity.
Key facts
- Atomic number
- 19
- Symbol
- K
- French name
- Potassium
- Atomic mass
- 39.098 u
- Molar mass
- 39.098 g/mol
- Family
- Alkali metals
- Position
- period 4, group 1, s-block
- State at 0 °C
- Solid
- Melting point
- 63.6 °C
- Boiling point
- 758.9 °C
- Density
- 0.86 g/cm³
- Electronegativity (Pauling)
- 0.82
- First ionization energy
- 419 kJ/mol
- Atomic radius
- 220 pm
- Electron configuration
- [Ar] 4s¹
- Abundance in Earth's crust
- 20,900 ppm (8th most abundant element)
- Indicative price
- 13 $/kg (order of magnitude)
- Radioactive
- no
- Discovery
- 1807, Humphry Davy
Origin of the name
From potash (literally “pot ash”); the symbol K comes from the Latin kalium.
What does potassium look like?
A very soft, silvery-white metal that tarnishes within seconds in air. Stored under mineral oil.
Where is potassium found?
About 2.1% of Earth's crust, in feldspars, micas and the evaporated salt beds of ancient seas. The human body contains about 140 g, mainly inside cells.
Natural sources
- Sylvite
- Carnallite
- Sylvinite
- Orthoclase (feldspar)
- Micas
How is potassium produced?
Potash salts are extracted from underground mines or by solution mining, then separated from ordinary salt. World production is around 40 million tonnes a year (expressed as potassium oxide). The pure metal, little used, is obtained by reducing the chloride with sodium.
Top producers
- Canada
- Russia
- Belarus
- China
What is potassium used for?
- Potash fertilizers (about 90% of production).
- Caustic potash for liquid soaps.
- Salt substitute (potassium chloride).
- Saltpeter (potassium nitrate) in black powder and fireworks.
- Specialty glass and the chemical industry.
In everyday life
- A banana (about 400 mg of potassium)
- Plant fertilizers
- Liquid soap
- Low-sodium salt
Role in living things
Essential: it is the main ion inside cells. The concentration difference between potassium (inside) and sodium (outside), maintained by a molecular pump, is what makes nerves and muscles work, the heart included. Plants need it to regulate their water.
Hazards of potassium
The metal reacts violently with water and can catch fire, with a lilac flame. In the blood, too much or too little potassium seriously disrupts heart rhythm. Natural potassium-40 is weakly radioactive, harmless at normal levels.
Cosmic origin
- Supernovae: Forged in the cores of massive stars, then scattered when they explode.
- White dwarfs: Produced by the thermonuclear explosion of a white dwarf (type Ia supernova).
History of potassium
In October 1807, Humphry Davy electrolyzed molten potash and saw globules of metal burst into flame: it was the first metal isolated using electricity, a few days before sodium. The name comes from “potash,” wood ashes leached in a pot. The symbol K comes from the Latin kalium, from the Arabic al-qaly, “plant ashes.”
Did you know?
- The potassium-40 in your body produces about 4,000 radioactive decays per second.
- Alsace, in eastern France, worked major potash mines from 1910 to 2002, discovered in 1904 during a search for oil.
- The “banana equivalent dose” is an informal unit used to illustrate small doses of radiation.
Isotopes of potassium
Two stable isotopes and one natural radioactive one, potassium-40. It is the main source of radioactivity in our own bodies, and it dates volcanic rocks.
- potassium-39: 93.258% of natural potassium, stable
- potassium-40: 0.012% of natural potassium, radioactive, half-life 1.25 × 10⁹ years. Potassium-argon dating of rocks and human fossils; it makes bananas slightly radioactive.
- potassium-41: 6.73% of natural potassium, stable
- potassium-42: radioactive, half-life 12.4 h. Tracer for studying potassium exchange in the heart and muscles.
Related elements
Same family (alkali metals): Lithium (Li), Sodium (Na), Rubidium (Rb), Caesium (Cs), Francium (Fr).
Same column (group 1): Hydrogen (H), Lithium (Li), Sodium (Na), Rubidium (Rb), Caesium (Cs), Francium (Fr).