Also available in: Português (Brasil)
Potassium (K), element 19
Potassium (K) is the chemical element with atomic number 19: an alkali metal in group 1, period 4. At 25 °C it is a solid, with an atomic mass of 39.1 u. Its electronegativity of 0.82 is among the lowest measured: it gives electrons away with extreme ease.
Physical and chemical data
| Atomic number | 19 |
|---|---|
| Symbol | K |
| Atomic mass | 39.1 u |
| Density | 0.89 g/cm³ |
| Melting point | 63.38 °C (336.53 K) |
| Boiling point | 758.85 °C (1,032 K) |
| Electronegativity (Pauling) | 0.82 |
| Electron configuration | [Ar]4s1 |
| Atomic radius (van der Waals) | 275 pm |
| Oxidation states | +1 |
| Physical state at 25 °C | solid |
| Family | alkali metal |
| Group and period | 1 / 4 |
| Discoverer and year | Humphry Davy (1807) |
The discovery record belongs to Humphry Davy, in 1807.
Neighbours on the table
Horizontally, one proton less lands on Argon (Z=18); one more lands on Calcium (Z=20). Vertically, group 1 gathers relatives with similar behaviour: Hydrogen, Lithium, Sodium, Rubidium.
The story behind the name
The name comes from potash, plant ash leached in pots and used as fertiliser and soap; the symbol K comes from the Latin kalium, inherited from the Arabic al-qalyah, plant ashes, the same root as the word alkali. It was the first metal ever isolated by electrolysis: in 1807 Humphry Davy drove the current of his giant battery through molten potash and watched metallic globules burst into lilac flames; by his brother's account, Davy danced around the laboratory in delight. Its natural radioactive isotope, potassium-40, makes every banana a tiny and harmless radiation source.
Physical and chemical data: PubChem Periodic Table (NCBI, compiling NIST and IUPAC), public domain, snapshot of 2026-07-10. Temperatures converted from kelvin; historical notes cross checked against classic chemistry references.
Last updated: · Methodology and sources