K · Element 19 · Alkali metal
A metal so reactive it must be kept away from air, and so essential that no cell in your body works without it.
Structure
Not a diagram of dots on rings — a Monte-Carlo sample of the actual probability density |ψ|² for each occupied subshell. Drag to rotate. Blue and violet mark opposite signs of the wavefunction, which is what makes bonding possible.
Measured values
Every bar shows where potassium sits among all 118 elements for that property.
Sources: IUPAC 2021 standard atomic weights · CRC Handbook of Chemistry and Physics · NIST. Values marked ~ are predicted rather than measured.
Same element, different neutrons
All potassium atoms have the same number of protons, 19, and differ only in their number of neutrons: those variants are its isotopes. The most common, K-39, has 20 neutrons and makes up 93.2581% of natural potassium.
| ISOTOPE | Neutrons | Natural Abundance | Half-life | Decay Mode |
|---|---|---|---|---|
| K-39 most common | 20 | 93.2581% | Stable | — |
| K-40 | 21 | 0.0117% | 1.248 × 10⁹ yrs | β⁻ (89.28%), ε/β⁺ (10.72%) |
| K-41 | 22 | 6.7302% | Stable | — |
Here the classroom shortcut, rounded atomic mass (39.098) minus atomic number, gives the same answer: 20.
Decay modes
Data: IAEA Nuclear Data Section, LiveChart of Nuclides, ground-state evaluations, retrieved October 10, 2026. Abundance is the share of atoms in the natural element; a dash means the isotope has no standard natural abundance.
Size, to scale
Radius 227 pm — that is 0.227 nm, so about 2.2 million of them side by side would span a millimetre.
Thermal range
Potassium is liquid over a 695 K window, from 337 K to 1032 K.
The story
Soft metal that reacts violently with water. Essential nutrient.
In 1807, a 28-year-old British chemist named Humphry Davy was obsessed with a new technique called electrolysis. Working at the Royal Institution in London, Davy had already gained fame for his dramatic public chemistry demonstrations, but he was about to make one of the most dangerous discoveries in scientific history.
Alessandro Volta's invention of the electric battery in 1800 had opened entirely new possibilities for chemistry. Davy realized that this powerful new tool might decompose substances that had resisted all previous attempts at analysis.
The target: potash (potassium carbonate), a mysterious alkali obtained from wood ashes that had puzzled chemists for centuries. Was it a compound or an element? Previous attempts using heat and chemical reactions had failed completely.
The Setup: Davy melted potash in a platinum spoon and inserted platinum wires connected to a powerful voltaic battery. As electric current flowed through the molten alkali, something unexpected happened at the negative electrode.
The Moment of Discovery: Tiny metallic spheres appeared, and some burst into flames with a violet light on contact with air. An early biography records that when Davy first saw the globules of potassium he bounded about the room in delight.
The Danger: These metallic globules were so reactive they exploded when they touched moisture in the air. Davy had isolated the first alkali metal - potassium - but handling it nearly cost him his life on multiple occasions.
Properties: Davy discovered that this new metal was lighter than water (it floated!), softer than wax (he could cut it with a knife), and so reactive it had to be stored under oil to prevent explosion.
The Name: Davy called it "potassium" from the English word "potash." Interestingly, most other languages use "kalium" (from Arabic "qali" meaning alkali), which is why the symbol is "K."
Immediate Impact: Just days later, Davy used the same technique to isolate sodium, proving that what chemists thought were simple substances were actually compounds of these very reactive metals.
Safety Struggles: For months, Davy and his assistants suffered burns and explosions while learning to handle potassium. They discovered it had to be cut under liquid paraffin and could never touch water (it explodes violently).
Legacy: Davy's discovery transformed chemistry by proving that electrical forces could decompose substances previously thought to be elemental. This opened the door to isolating many other elements and established electrochemistry as a fundamental science.
Napoleon's Prize: Even though Britain and France were at war, Napoleon's Institut de France awarded Davy its galvanism prize in 1807 for his 1806 Bakerian Lecture on electrochemistry.
Modern Relevance: Davy's dangerous experiments laid the foundation for modern electrochemistry, including rechargeable batteries, electroplating, and aluminum production - all multi-billion dollar industries today.
Applications
Potassium is essential for plant growth and forms the backbone of modern agriculture. As one of the three primary macronutrients (N-P-K), potassium regulates water balance, enzyme activation, and photosynthesis in plants.
Fertilizer Manufacturing Process: Potassium chloride (KCl) is extracted from underground deposits through solution mining. Heated water dissolves the salt deposits, creating a brine that is pumped to the surface and evaporated in massive crystallization ponds. The resulting pink-tinted crystals are 95-99% pure KCl.
Forms Used: Potassium sulfate (K₂SO₄) for chloride-sensitive crops like tobacco and potatoes, potassium nitrate (KNO₃) for high-value crops requiring both potassium and nitrogen, and potassium phosphate for precision agriculture.
Soap and Detergent Production: Potassium hydroxide (KOH), known as caustic potash, is manufactured through the electrolysis of potassium chloride brine. This process creates KOH at the cathode and chlorine gas at the anode. KOH produces softer, more soluble soaps compared to sodium-based alternatives.
Glass Industry: Potassium carbonate (K₂CO₃) acts as a flux in glass manufacturing, lowering the melting point of silica and creating harder, more brilliant glass. This is why high-quality crystal glassware often contains potassium instead of sodium.
Gunpowder Manufacturing: Potassium nitrate (saltpeter) serves as the oxidizer in black powder, providing oxygen for combustion. Modern smokeless powders still use potassium compounds to control burn rates and reduce muzzle flash.
Fireworks Chemistry: Potassium compounds supply oxygen rather than color - potassium chlorate and potassium perchlorate serve as oxidizers, while the colors come from other metal salts such as strontium (red) and copper (blue). The potassium ion itself burns with a characteristic violet flame.
Heat Transfer Systems: Liquid potassium-sodium alloys (NaK) are used as coolants in nuclear reactors and space applications due to their excellent heat transfer properties and low vapor pressure at high temperatures.
Photography: Potassium bromide and potassium ferricyanide are essential in traditional black-and-white photo processing, controlling contrast and enabling precise image development.
Where it comes from
20900 mg/kg of Earth's crust · more abundant than 94% of elements
Potassium ranks as the 8th most abundant element in Earth's crust (2.09% by weight), slightly less common than sodium (2.36%). This abundance reflects potassium's role in the fundamental rock-forming minerals that built our planet.
Primary Mineral Sources:
Saskatchewan, Canada: Contains world's largest potash reserves, formed 400 million years ago when the Williston Basin was a shallow sea. The Esterhazy mine extends 1 kilometer underground.
Dead Sea, Israel/Jordan: Brine contains about 0.6% potassium, extracted through solar evaporation ponds that create a rainbow of colors from different mineral concentrations.
Ural Mountains, Russia: Ancient evaporite beds provide 20% of world production, with some mines operating since the Soviet era.
Carlsbad, New Mexico: The Permian Basin contains extensive potash beds formed when ancient seas evaporated 250 million years ago.
Seawater Concentration: Modern oceans contain about 380 parts per million of potassium, maintained through the balance between river input and uptake by marine organisms and clay minerals.
Great Salt Lake: Contains concentrated potassium brines that are commercially extracted alongside sodium and magnesium.
Nucleosynthesis: Potassium-39 forms mainly through explosive oxygen burning in massive stars. The process requires temperatures exceeding 1.5 billion Kelvin.
Supernova Production: Potassium-40, the radioactive isotope, is created during explosive nucleosynthesis in supernovae and contributes to Earth's internal heat through radioactive decay.
Meteorites: Chondritic meteorites contain potassium in feldspar minerals, providing clues about the early solar system's composition and the formation of rocky planets.
Soil Formation: Weathering of feldspar and mica releases potassium ions that become available to plants. Clay minerals can trap and slowly release this potassium over time.
Plant Uptake: Root systems actively transport potassium against concentration gradients, concentrating it 1000-fold compared to soil solutions. This process requires significant metabolic energy.
Handling
Risk Level: HIGH - Potassium metal is extremely dangerous and requires specialized handling procedures.
Potassium Metal: Must be stored under mineral oil or kerosene in sealed, inert atmosphere containers. Never store near water sources or oxidizing agents.
Potassium Compounds: Store in dry, cool areas away from acids and oxidizers. Keep containers tightly sealed to prevent moisture absorption.
Separation Requirements: Keep potassium compounds away from ammonium salts, which can form explosive mixtures when heated.
OSHA PEL (Permissible Exposure Limit): No specific limit for potassium metal (too reactive for routine exposure)
Dietary Potassium: Adequate intake is 3,400 mg daily for men and 2,600 mg for women. Average intakes are below these amounts.
Toxicity Threshold: Hyperkalemia (too much potassium) occurs above 5.5 mEq/L in blood, can cause heart rhythm abnormalities
Skin Contact with Metal: Wipe or brush off any metal immediately, then flush with running water for at least 20 minutes. Remove contaminated clothing. Seek immediate medical attention for burns.
Eye Contact: Flush eyes with water for 15+ minutes, holding eyelids open. Remove contact lenses if possible. Get immediate medical attention.
Inhalation of Dust/Fumes: Move victim to fresh air immediately. If breathing difficulties occur, provide oxygen and seek medical attention.
Ingestion: Do NOT induce vomiting. Rinse mouth with water. Give small amounts of water to drink. Seek immediate medical attention.
Potassium Fires: NEVER use water. Use Class D fire extinguisher (dry sand, sodium chloride, or specialized metal fire suppressants)
Spill Response: Evacuate area. Ventilate space. Cover spilled potassium with dry sand or oil. Do not allow any moisture contact.
Explosion Risk: If potassium contacts water, explosive hydrogen gas forms. Eliminate all ignition sources immediately.
Quick answers
Potassium (symbol K) is element 19 on the periodic table, an alkali metal in period 4, group 1. A metal so reactive it must be kept away from air, and so essential that no cell in your body works without it. At room temperature it is a solid.
[Ar] 4s¹, giving 4 occupied shells with the distribution 2, 8, 8, 1.Potassium melts at 336.5 K (63.4 °C) and boils at 1032 K (758.9 °C).
The standard atomic weight of potassium is 39.098 u. That is a weighted average across its naturally occurring isotopes, which is why it is rarely a whole number.
The most common potassium atom, K-39, has 20 neutrons: its mass number, 39, minus its atomic number, 19. It makes up 93.2581% of natural potassium. The classroom shortcut, rounded atomic mass (39.098) minus atomic number, gives the same 20.
Potassium has a density of 0.89 g/cm³. Water is 1.0 g/cm³, so a block of potassium floats.
Potassium has a Pauling electronegativity of 0.82. The scale runs from 0.70 (francium, the least greedy for electrons) to 3.98 (fluorine, the most). A value this low means it readily gives its outer electrons away, forming positive ions.
Potassium was discovered in 1807 by Humphry Davy. Origin of the name: English potash; symbol from Latin kalium.
Potassium makes up about 2.09% of the Earth's crust — one of the most abundant elements there is.