Potassium (K)
Potassium (K) is an alkali metal with atomic number 19 and a standard atomic weight of 39.098. At room temperature it is a solid that melts at 64 °C and boils at 759 °C.
Potassium key facts
Potassium at a glance
Identity
| Symbol | K |
|---|---|
| Atomic number (Z) | 19 |
| Atomic weight | 39.098 ± 0.001 (CIAAW 2024) |
| Category | Alkali metal |
| Group | 1 — alkali metals (with hydrogen) |
| Period | 4 |
| Block | s-block |
| CAS number | 7440-09-7 |
Atomic properties
| Electron configuration | [Ar] 4s1 Full: 1s2 2s2 2p6 3s2 3p6 4s1 |
|---|---|
| Electrons per shell | 2, 8, 8, 1 |
| Oxidation states | +1 Less common: -1 |
| Electronegativity (Pauling) | 0.82 |
| First ionisation energy | 4.341 eV (418.8 kJ/mol) 2nd: 31.625 eV · 3rd: 45.803 eV |
| Electron affinity | 0.501 eV |
| Atomic radius (empirical) | 220 pm |
| Covalent radius | 196 pm |
| Van der Waals radius | 275 pm |
Physical properties
| State at 25 °C | Solid |
|---|---|
| Melting point | 63.5 °C · 146.3 °F · 336.65 K |
| Boiling point | 759 °C · 1,398.2 °F · 1,032.15 K |
| Density | 0.89 g/cm³ |
| Specific heat capacity | 0.757 J/(g·K) |
| Thermal conductivity | 79 W/(m·K) |
| Crystal structure | Body-centred cubic |
Isotopes & abundance
| Natural occurrence | Primordial |
|---|---|
| Radioactive | No |
| Natural isotopes (abundance) | 39K 93.2581%, 41K 6.7302%, 40K 0.0117% (radioactive) |
| Stable isotopes | 5 |
| Known isotopes | 29 |
| Abundance in Earth's crust | 20,900 mg/kg |
| Abundance in seawater | 399 mg/L |
Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.
Potassium electron configuration
Potassium has 19 electrons. Written in short form, its ground-state configuration is [Ar] 4s1 — the electrons of the noble gas argon, plus 4s1.Written out in full: 1s2 2s2 2p6 3s2 3p6 4s1.
As a member of group 1 it has 1 valence electron in its outer shell (shell 4), which largely decides its chemistry.
Discovery and name
1807 — Humphry Davy · England.
English 'potash'; the symbol K is from Latin kalium.
Known for
A soft metal that reacts violently with water. Potassium salts (potash) are major fertilisers; the K⁺ ion is vital for nerve signals and heart rhythm.
Isotopes of potassium
Naturally occurring potassium is a mix of 3 isotopes. Each has 19 protons; they differ in the number of neutrons:
| Isotope | Protons | Neutrons | Natural abundance | Stability |
|---|---|---|---|---|
| 39K | 19 | 20 | 93.2581% | stable |
| 41K | 19 | 22 | 6.7302% | stable |
| 40K | 19 | 21 | 0.0117% | radioactive · half-life 1.248 billion years |
29 isotopes of potassium are known in our data set (half-lives as published in nuclear data tables).
Potassium compared with group 1
The other elements in group 1 (alkali metals (with hydrogen)), from top to bottom of the table:
| Element | Z | Atomic weight | Melting point | Boiling point | Density | Electronegativity |
|---|---|---|---|---|---|---|
| Hydrogen | 1 | 1.0080 | -259 °C | -253 °C | 0.0899 g/L | 2.2 |
| Lithium | 3 | 6.94 | 180 °C | 1,342 °C | 0.534 g/cm³ | 0.98 |
| Sodium | 11 | 22.990 | 98 °C | 883 °C | 0.97 g/cm³ | 0.93 |
| Potassium | 19 | 39.098 | 64 °C | 759 °C | 0.89 g/cm³ | 0.82 |
| Rubidium | 37 | 85.468 | 39 °C | 688 °C | 1.53 g/cm³ | 0.82 |
| Cesium | 55 | 132.91 | 28 °C | 671 °C | 1.873 g/cm³ | 0.79 |
| Francium | 87 | [223] | 21 °C (est.) | — | 1.87 g/cm³ (pred.) | 0.7 |