Potassium

Potassium (K) is an alkali metal with atomic number 19 and an atomic mass of 39.098. Its electron configuration is [Ar] 4s1, which gives it 1 valence electron.

Potassium key data

Symbol	K
Atomic number	19
Atomic mass (standard atomic weight)	39.098
Molar mass	39.098 g/mol
Protons	19
Neutrons (most abundant isotope, potassium-39)	20
Electrons (neutral atom)	19
Valence electrons	1
Category	Alkali metal
Period	4
Group	1
Block	s-block
Noble gas notation	[Ar] 4s1
Full electron configuration	1s2 2s2 2p6 3s2 3p6 4s1
Electrons per shell	2, 8, 8, 1

Atomic mass: IUPAC CIAAW abridged standard atomic weights, 2024. Most abundant isotope: NIST Atomic Weights and Isotopic Compositions.

Measured properties of potassium

From PubChem, the US National Institutes of Health. Public domain.

Melting point	336.53 K (63.38 °C), 85th of 103 by melting point
Boiling point	1032 K (758.85 °C), 73rd of 93 by boiling point
Density	0.89 g/cm³, 84th of 96 by density
State at room temperature	Solid
Electronegativity	0.82 (Pauling), joint 92nd of 95 by electronegativity
Van der Waals radius	275 pm, 5th of 99 by atomic radius
First ionisation energy	4.341 eV (418.8 kJ/mol), 99th of 102 by first ionisation energy
Electron affinity	0.501 eV (48.3 kJ/mol), 39th of 57 by electron affinity
Common oxidation states	+1
Discovered	1807

Each rank counts down from the highest value, among the elements PubChem’s table gives one for, and links to that property ranked across the whole table. All of them are collected under periodic trends.

Potassium electron configuration

The full electron configuration of potassium is 1s2 2s2 2p6 3s2 3p6 4s1. With the noble-gas core collapsed it is [Ar] 4s1, and the outermost subshell is 4s1.

Bohr model of potassium (K): a nucleus and four shells holding 2, 8, 8 and 1 electrons from the inside out. — Bohr model of potassium (K). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Orbital box diagram of potassium (K), 1s2 2s2 2p6 3s2 3p6 4s1: one box per orbital, filled by Hund’s rule, showing one unpaired electron, in 4s. — Orbital diagram of potassium (K). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Why group and block follow from this is set out in how the periodic table maps electron configuration, and the electron configuration of every element is listed on one page.

Mass of one potassium atom

A molar mass is grams per mole, so dividing by the Avogadro constant gives the mass of a single atom, or for an element with more than one natural isotope the average mass of its atoms, and the reciprocal gives how many atoms sit in a gram.

Mass of one atom

one atom = 39.098 / (6.02214076 × 10²³)
= 6.4924 × 10⁻²³ g

Atoms in one gram

atoms per gram = 6.02214076 × 10²³ / 39.098
= 1.5403 × 10²² atoms

Potassium on the periodic table

Potassium is in period 4, group 1, in the s-block.

The other elements of group 1 are hydrogen, lithium, sodium, rubidium, caesium and francium.