Potassium
Atomic Data
| Atomic Number | 19 |
| Symbol | K |
| Atomic Weight | 39.098 u |
| Density (STP) | 0.862 g/cm³ |
| Melting Point | 63.38 °C (336.53 K) |
| Boiling Point | 758.85 °C (1032 K) |
| Electronegativity | 0.82 (Pauling) |
| Electron Config. | 1s2 2s2 2p6 3s2 3p6 4s1 |
| Oxidation States | +1 |
| Phase at STP | Solid |
| Category | Alkali Metal |
| Period / Group | 4 / 1 |
| CAS Number | 7440-09-7 |
Electron Configuration
[Ar] 4s1
| Shell | n | Subshell | Electrons | Cumulative |
|---|---|---|---|---|
| K | 1 | 1s | 2 | 2 |
| L | 2 | 2s | 2 | 4 |
| L | 2 | 2p | 6 | 10 |
| M | 3 | 3s | 2 | 12 |
| M | 3 | 3p | 6 | 18 |
| N | 4 | 4s | 1 | 19 |
| Total | 19 | 19 | ||
Isotopes of Potassium
Potassium has three naturally occurring stable isotopes. The most abundant is ³⁹K, comprising 93.2581% of all naturally occurring Potassium.
| Isotope | Symbol | Protons | Neutrons | Abundance | Stability |
|---|---|---|---|---|---|
| Potassium-39 | ³⁹K | 19 | 20 | 93.2581 | Stable |
| Potassium-40 | ⁴⁰K | 19 | 21 | 0.0117 | Stable |
| Potassium-41 | ⁴¹K | 19 | 22 | 6.7302 | Stable |
Abundance & Occurrence
Potassium is present in Earth's crust at approximately 20900 ppm by mass and at approximately 3 ppm by mass throughout the universe.
Earth's Crust (ppm by mass)
Universe (ppm by mass)
Discovery & History
Read more about the discovery of the periodic table of elements →
Safety & Handling
- Violent water reactivity: Potassium reacts explosively with water, generating hydrogen gas and potassium hydroxide; even small quantities of potassium in contact with moisture produce immediate ignition of the hydrogen, and larger pieces can detonate.
- Spontaneous ignition: Potassium metal oxidises rapidly in air; freshly exposed surfaces develop a grey oxide crust, and oxidised potassium superoxide (KO2) can form spontaneously — a highly reactive, shock-sensitive substance.
- Severe burns: Contact with potassium metal or potassium hydroxide solution causes deep alkaline burns; potassium hydroxide solution is particularly destructive to eye tissue and causes corneal damage within seconds.
- Fire hazard: Potassium fires cannot be extinguished with water or CO2; use dry sand or Class D extinguisher only. Never use water — the explosive reaction will spread burning potassium.
- Storage: Store under mineral oil in sealed containers, away from water, moisture, and oxidisers; inspect stored potassium regularly for oxidation crust, which must be safely disposed of — do not allow heavy oxide layers to accumulate.
Potassium in the Real World
Real-World Uses
- Fertilisers — Potassium chloride (muriate of potash), potassium sulfate, and potassium nitrate are applied to agricultural soils worldwide to supply the potassium essential for plant growth, water regulation, and disease resistance.
- Food processing — Potassium chloride is used as a sodium-reduced salt substitute and in sports drinks; potassium sorbate and potassium benzoate are common food preservatives that inhibit mould and yeast growth.
- Medicine and intravenous therapy — Potassium chloride solutions correct hypokalaemia (low blood potassium), which can cause dangerous cardiac arrhythmias; potassium is a major component of balanced IV electrolyte infusions.
- Explosives and pyrotechnics — Potassium nitrate (saltpetre) is the oxidiser in black gunpowder and in pyrotechnic compositions; potassium perchlorate is widely used in fireworks, flares, and airbag inflators.
- Water softening — Potassium chloride is used as an alternative to sodium chloride to regenerate domestic ion-exchange water softeners, reducing sodium in treated water for health-conscious consumers.
- Soap making — Potassium hydroxide (caustic potash) saponifies fats and oils to produce soft soaps, shaving creams, and liquid hand soaps, in contrast to sodium hydroxide which yields solid bar soaps.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
What is potassium used for?
Potassium's major use is as potash fertilisers (potassium chloride, potassium sulfate) that replenish soil potassium depleted by crop harvesting. In medicine, potassium chloride corrects dangerous low-blood-potassium states. Potassium nitrate is used in gunpowder, fireworks, and food preservation. Potassium hydroxide is used to make soft soaps and in alkaline batteries. Potassium is also an essential electrolyte in the human body.
Why is potassium's symbol K?
The symbol K comes from kalium, the Neo-Latin name for potassium, which was derived from the Arabic word 'al-qali' (the plant ash), the historical source of potassium carbonate. The English name 'potassium' comes from potash, a general term for various potassium salts extracted from wood ash and plant matter. Humphry Davy, who first isolated potassium by electrolysis in 1807, used the name 'potassium', but the continental European name kalium (used by Berzelius) gave us the symbol K.
Is potassium essential for the body?
Yes, potassium is an essential mineral for all animal life. It is the major intracellular cation, maintained at high concentration inside cells relative to sodium outside. This concentration gradient, maintained by the sodium-potassium pump (Na⁺/K⁺-ATPase), is fundamental to nerve impulse transmission, muscle contraction (including heartbeat), and regulation of cell volume and pH. Potassium deficiency (hypokalaemia) causes muscle weakness, cramps, constipation, and dangerous cardiac arrhythmias.
Is potassium radioactive?
Naturally occurring potassium contains a small fraction (0.0117%) of potassium-40 (K-40), which is radioactive with a half-life of 1.25 billion years. K-40 decays by emitting a beta particle (to calcium-40) or by electron capture (to argon-40), making it a minor source of natural background radiation in food, in the human body, and in rocks. The amount of radiation from potassium in food is harmless, but K-40 decay is an important source of radiogenic heat inside the Earth.