Potassium (K)
Element 19 · Alkali metal · atomic mass 39.098 u · group 1, period 4
Electron shells: 2, 8, 8, 1
Potassium is so reactive it ignites on contact with water, yet its ion is essential in every living cell. It is the main nutrient after nitrogen in agricultural fertiliser.
You are naturally slightly radioactive: about 0.012% of your potassium is potassium-40.
- Atomic number
- 19
- Atomic mass
- 39.098 u
- Electron configuration
- [Ar] 4s¹
- Group · period · block
- 1 · 4 · s
- Oxidation states
- +1
- Electronegativity
- 0.82 (Pauling)
- Melting point
- 63.38 °C
- Boiling point
- 759 °C
- Density
- 0.89 g/cm³
- State at 20 °C
- Solid
- Appearance
- Soft, silvery-white metal
- Discovered
- Humphry Davy, 1807
19 protons, and 19 electrons when neutral
IUPAC standard atomic weight
Alkali metals
What Potassium is used for
- Potash fertilisers
- Glass manufacture
- Soaps and detergents
- Salt substitutes
Atomic structure of Potassium
A neutral potassium atom has 19 protons and 19 electrons. Its most common isotope carries about 20 neutrons, which is where the mass number 39 comes from. Change the neutron count and you have a different isotope of potassium; change the proton count and you have a different element entirely.
Those electrons fill shells from the inside out: 2, 8, 8, 1. In orbital notation that is [Ar] 4s¹, leaving 1 valence electron in the outer shell — the ones that do the chemistry.
3D electron orbitals
See where Potassium's electrons actually are — a cloud of 26,000 points sampled from the probability density |ψ|², not a cartoon of a lobe.
Where Potassium sits in the periodic table
Group 1 metals with a single outer electron. Soft, low-melting and so reactive they are stored under oil — reactivity increases as you go down the group.
Discovery and naming
Potassium was discovered by Humphry Davy in 1807. Its name comes from English potash; symbol from Latin kalium, which is also why its symbol (K) does not match its English name.
Keep exploring
Atomic weight from IUPAC/CIAAW; physical properties from PubChem (US National Library of Medicine). Values that have never been measured are shown as “not measured” rather than estimated.
Related Calculators
Frequently Asked Questions
What is the atomic mass of Potassium?
The standard atomic weight of Potassium is 39.098 u, as published by IUPAC's Commission on Isotopic Abundances and Atomic Weights.
How many protons, neutrons and electrons does Potassium have?
Potassium has 19 protons, because the atomic number is the proton count and that is what makes an atom Potassium at all. A neutral Potassium atom has the same number of electrons, 19. The neutron count varies by isotope; the most common one has about 20, since 39 − 19 = 20.
What is the electron configuration of Potassium?
The electron configuration of Potassium is [Ar] 4s¹. Its electrons are arranged 2, 8, 8, 1 across 4 shells.
Is Potassium a metal?
Yes. Potassium is classified as a alkali metal, on the left side of the table.
What group and period is Potassium in?
Potassium is in group 1 (the alkali metals), period 4, in the s-block. The group is its column and the period is its row.
What is the charge of Potassium?
Potassium most commonly forms a 1+ ion. It has 1 electron in its outer shell and loses it readily, which is what the group number tells you.
What is the melting point of Potassium?
Potassium melts at 63.38 °C and boils at 759 °C. That puts it in the solids at everyday temperatures.
What is Potassium used for?
Potash fertilisers, Glass manufacture, Soaps and detergents, Salt substitutes. Potassium is so reactive it ignites on contact with water, yet its ion is essential in every living cell.
Who discovered Potassium, and where does its name come from?
Potassium (K) was discovered by Humphry Davy in 1807. The name comes from English potash; symbol from Latin kalium.