ElementsWhat is potassium?
Potassium is a soft, silvery metal that reacts violently with water. Plants and animals need its salts. Nerves and muscles cannot work without it.
What is potassium used for?
Fertilisers take most of it. Also soaps, glass, and food salt substitutes.
Where the name comes from
From potash, the ashes of burnt plants. The symbol K comes from kalium, its Latin name.
Key facts
- Symbol
- K
- Atomic number
- 19
- Atomic mass
- 39.0983 u
- Family
- Alkali metal
- Period
- 4
- Group
- 1
- Block
- s
- State at room temperature
- Solid
- Melting point
- 63.4 °C (336.53 K, 146 °F)
- Boiling point
- 758.9 °C (1,032 K, 1,398 °F)
- Density
- 0.89 g/cm³
- Electrons per shell
- 2, 8, 8, 1
- Electron configuration
- [Ar]4s1
- Common oxidation states
- +1
- Electronegativity (Pauling)
- 0.82
- First ionisation energy
- 4.341 eV
- Electron affinity
- 0.501 eV
- Atomic radius (van der Waals)
- 275 pm
- Discovered
- 1807
Place in the periodic table
Potassium is element number 19. It sits in period 4 (row 4) of the periodic table, in group 1 (column 1), and belongs to the s-block. Just before it comes argon (18), and just after it comes calcium (20). In the same column, sodium is above it and rubidium is below it. Elements in one column tend to behave alike.
Family: Alkali metals.
Physical properties
At room temperature potassium is a solid. It melts at 63.4 °C and boils at 758.9 °C, so it stays liquid over a range of 695 degrees. Its density is 0.89 grams per cubic centimetre, so a one-litre block would weigh about 0.89 kilograms. It is lighter than water and would float, if it did not react with it.
Inside the atom
An atom of potassium has 19 protons in its nucleus and, when neutral, 19 electrons around it. The electrons are arranged in 4 shells: 2, 8, 8, 1. The outermost shell holds 1 electron. Written in full, the electron configuration is [Ar]4s1.
Its electronegativity on the Pauling scale is 0.82. This number shows how strongly an atom pulls shared electrons towards itself: fluorine, the strongest, is 3.98. Removing one electron from an atom takes 4.341 electronvolts (about 419 kilojoules per mole). One mole of potassium weighs 39.0983 grams, so a single gram contains about 1.54 × 1022 atoms.
The ring picture is a simple model that only counts the electrons in each shell. Real atoms do not look like this.
Next steps
- Work out the weight of a compound that contains potassium with the molar mass calculator.
- Compare it with every other element in the sortable list.
- Test yourself with the symbol quiz.
Figures on this page are from PubChem (U.S. National Library of Medicine), as downloaded on 11 November 2026. See About and data sources.