Potassium
Potassium is a chemical element with symbol K (derived from Neo-Latin, kalium) and atomic number 19. It was first isolated from potash, the ashes of plants, from which its name is derived. In the Periodic table, potassium is one of seven elements in column (group) 1 (alkali metals):they all have a single valence electron in their outer electron shell, which they readily give up to create an atom with a positive charge - a cation, and combine with anions to form salts.
Identity
- Atomic mass
- 39.098 u
- Category
- Alkali metals
- Group
- 1
- Period
- 4
- Block
- s
- Oxidation States
- +1
- Phase (25 °C)
- Solid
- Appearance
- silvery gray
Electronic Structure
- Electron configuration
- 1s2 2s2 2p6 3s2 3p6 4s1
- Short configuration
- [Ar] 4s1
- Electrons per shell
- 2, 8, 8, 1
- Electronegativity (Pauling)
- 0.820
- Electron affinity
- 48.4 kJ/mol
- 1st ionization energy
- 419 kJ/mol
- Ionization energies
- 419 · 3052 · 4420 · 5877 · 7975… kJ/mol
Physical properties
- Melting point
- 63.6 °C (337 K)
- Boiling point
- 758.9 °C (1032 K)
- Density
- 0.862 g/cm³
- Molar heat capacity
- 29.6 J/(mol·K)
Chemical behavior of potassium
Potassium has a single electron in the 4s orbital ([Ar] 4s¹), retained with even less force than in sodium due to greater electronic shielding. Therefore, it immediately cedes this electron to form the K⁺ cation with the stable electron configuration of the noble gas argon. It originates almost exclusively ionic bonds with non-metals and is an extremely electropositive metal. As a trend within the alkali metal group, it is notably more reactive and softer than sodium, and shows a strong preference for forming superoxides rather than simple oxides.
Reactivity
2 K + 2 H₂O → 2 KOH + H₂
Reacts violently with liquid water releasing hydrogen; the heat released ignites the gas instantaneously, producing a characteristic lilac or violet flame.
2 K + Cl₂ → 2 KCl
Potassium metal combines vigorously with chlorine gas to form potassium chloride in a strongly exothermic reaction.
K + O₂ → KO₂
When burning in air or atmosphere with excess oxygen, it preferably forms potassium superoxide, unlike lithium or sodium which form oxide or peroxide.
2 K + 2 C₂H₅OH → 2 C₂H₅OK + H₂
Reacts with alcohols such as ethanol releasing hydrogen gas and forming potassium ethoxide, a strong organic base.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Potassium chloride | KCl | Essential agricultural fertilizer, medical supplement and dietary substitute for table salt |
| Potassium hydroxide | KOH | Caustic potash; soft soap manufacturing, electrolyte in alkaline batteries and paint removers |
| Potassium nitrate | KNO₃ | Saltpeter; soluble nitrogen fertilizer, pyrotechnics and traditional food preservative |
| Potassium carbonate | K₂CO₃ | Optical and laboratory glass manufacturing, ceramics and industrial acidity regulator |
| Potassium superoxide | KO₂ | Chemical oxygen generator and CO₂ scrubber in emergency respirators and submarines |
Electron shell distribution
View in the atom builderIsotopes
Natural potassium is made up of three isotopes: two stable ones, potassium-39 (³⁹K) with 20 neutrons (93.26 %) and potassium-41 (⁴¹K) with 22 neutrons (6.73 %), and a primordial radionuclide of very long half-life, potassium-40 (⁴⁰K, 0.012 %, with a half-life of 1.25 billion years). ⁴⁰K generates most of the natural radioactivity emitted by the human body and is used extensively in geology for dating ancient volcanic rocks through the potassium-argon method.
Occurrence and production
It is the seventh most abundant element in the Earth's crust (2.6 % by mass), but it is never found in the free state due to its high reactivity. It is concentrated in marine evaporite saline deposits in the form of minerals such as sylvite, carnallite and polyhalite, in addition to being present in feldspars and micas. The obtaining of the elemental metal is not done by aqueous electrolysis, but by thermal reduction of molten KCl with sodium vapor at about 870 °C.
History and origin of the name
It was isolated for the first time in 1807 by the British chemist of molten potassium hydroxide (caustic potash), constituting the first metallic element isolated by electrochemical means. Its popular name derives from the Dutch and English term "potash" (pot ash), which alluded to wood ashes boiled in iron vessels to extract potassium carbonate. The universal chemical symbol K comes from the scientific Latin "kalium", taken in turn from the Arabic word "al-qalyah" (calcined ashes of plants).
Applications and relevance of potassium
Uses of the element
Pure metallic potassium has restricted demand due to its high danger and reactivity. Its main use is the sodium-potassium eutectic alloy (NaK), which is liquid at room temperature and functions as a high-conductivity heat transfer refrigerant in fast nuclear reactors and specialized thermal circuits. It is also used in laboratories as a reducing agent in organic synthesis and in the production of potassium superoxide.
Uses of its compounds
More than 90 % of the world's potassium is destined for agriculture in the form of potassium chloride, sulfate or nitrate, vital nutrients for plant vigor and water stress tolerance. Potassium hydroxide is used in alkaline batteries and liquid soaps; carbonate in fine glassware and enamels; and mixed sodium and potassium tartrate (Seignette's salt) in acoustic components and confectionery.
Health, safety and environment
Biological role
The K⁺ cation is the predominant cation within the cytoplasm of living cells. It is essential to sustain the transmembrane electrical potential, enable depolarization and nerve transmission, coordinate skeletal muscle contraction and ensure regular cardiac function. Alterations in its plasma levels cause serious failures in myocardial conduction and neuromuscular weakness.
Safety
The metal is extremely dangerous: it reacts violently with water, environmental humidity and even with ice, instantly inflaming the hydrogen generated. It forms surface peroxides and superoxides that can detonate on cutting or impact. It is preserved under dried inert hydrocarbons and its fires must be extinguished only with Class D dry powder extinguishers, never with water or carbon dioxide extinguishers.
Environment
Potassium is a primordial macronutrient for terrestrial and aquatic ecosystems. It is not toxic at usual concentrations, but massive agricultural runoff of potassium fertilizers and nitrates can favor saline and osmotic imbalances in receiving rivers and lakes if soil-adjusted doses are not applied.
Frequently asked questions about Potassium
Why does potassium emit a violet flame when burning?
When heat is supplied to potassium, its outermost electron absorbs thermal energy and jumps to a higher orbital level. When the excited electron returns almost immediately to its ground state, it releases that absorbed energy in the form of light photons. The dominant wavelength of this emission falls in the violet and bluish zone of the visible spectrum. This characteristic light emission makes it easy to identify potassium salts in laboratory analysis.
Why does regular banana consumption not pose a radiation risk due to its potassium-40?
Although the banana contains potassium and a tiny fraction of it is the radioactive isotope potassium-40, the human body does not accumulate unlimited radioactivity. The organism maintains strict homeostasis: it regulates potassium levels by absorbing only what is necessary and excreting the excess through urine within a few hours. The natural radiation dose of a banana is minuscule, perfectly tolerable and incapable of altering the internal radiation balance of the human body.
How can scientists determine the age of a rock using potassium?
Some rocks contain potassium-40, a radioactive isotope that slowly transforms into argon-40. When volcanic rock forms, the argon that was there before usually escapes, but the argon produced afterward can remain trapped inside. Scientists measure the amount of potassium-40 remaining and the accumulated argon to estimate how much time has passed since the rock cooled. This method allows studying geological events that occurred millions or even billions of years ago.
Why do plants need potassium to regulate water loss through leaves?
Leaves have small pores called stomata that allow absorbing carbon dioxide and releasing water vapor. Potassium helps control when these pores open and close. When it accumulates in the cells surrounding them, it favors water entry and the stomata open. When these cells lose potassium and water, the pores close, reducing moisture loss. Therefore, adequate potassium nutrition is important for plant growth and drought resistance.
Why does mixing potassium and sodium produce a liquid metal?
Although potassium and sodium are solid metals separately, when combined they form an alloy called NaK that becomes liquid at room temperature. Their atoms are so different in size that they cannot fit into a regular solid crystal lattice. This property allows it to be used as a special refrigerant in nuclear reactors and space satellites.
Why is potassium essential for muscles and nerves to function?
Potassium is essential for nerve impulse transmission in the human body. Cells maintain a high concentration of potassium inside and sodium outside. This difference generates a small electrical charge across the cell membrane that allows the heart to beat regularly and muscles to contract properly.
Why does potassium chloride leave a slightly metallic or bitter taste in dietary salt?
Potassium chloride is used as a common salt substitute for people with hypertension because it provides a salty taste without increasing sodium. However, while the sodium ion fits perfectly into the salty taste receptors of the human tongue, the potassium cation is larger. This difference makes it simultaneously stimulate bitter taste receptors and leave a metallic aftertaste, reason why the food industry usually combines it with herbs or enhancers.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.