Element: Potassium

General data

Element name	Potassium
Chemical symbol	K
Atomic number Z	19
Year discovered	1807
Standard state (25°C)	Solid
Category	Alkali metal
Block	s
Atomic mass	39.098
CPK color	#8F40D4

Physical properties

Melting point	336.53 K (63.38 °C)
Boiling point	1032 K (759 °C)
Density	0.89 g/cm³

Energy properties

First ionization energy	4.341 eV
Electron affinity	0.501 eV

Atomic structure

Protons	19 (Atomic number)
Neutrons (K-39, most abundant isotope)	20
Electrons	19
Electron configuration	[Ar] 4s¹
Oxidation states	+1
Electronegativity (Pauling)	0.82
Atomic radius (van der Waals)	275 pm

Ionization energies - NIST ASD (19 states)

State	Charge	Ioniz. energy (eV)	Type
K I	0+	4.34066373 ± 0.00000009	measured
K II	1+	31.625 ± 0.00019	measured
K III	2+	45.8031 ± 0.0017	semi-empirical
K IV	3+	60.917 ± 0.019	semi-empirical
K V	4+	82.66 ± 0.16	semi-empirical
K VI	5+	99.44 ± 0.2	semi-empirical
K VII	6+	117.56 ± 0.11	measured
K VIII	7+	154.87 ± 0.05	semi-empirical
K IX	8+	175.8174 ± 0.0025	semi-empirical
K X	9+	503.67 ± 0.07	semi-empirical
K XI	10+	565.6 ± 0.7	semi-empirical
K XII	11+	631.1 ± 0.9	semi-empirical
K XIII	12+	714.7 ± 0.5	semi-empirical
K XIV	13+	786.3 ± 0.3	semi-empirical
K XV	14+	860.92 ± 0.25	semi-empirical
K XVI	15+	967.7 ± 0.3	semi-empirical
K XVII	16+	1034.542 ± 0.007	calculated
K XVIII	17+	4610.80714 ± 0.00025	calculated
K XIX	18+	4934.04979 ± 0.00005	calculated

Value type as given by NIST ASD: measured - from evaluated experimental data; calculated - from ab initio calculations (mostly highly ionised atoms that have not been measured; for the simplest ones, e.g. H I, the calculation is more accurate than measurement); semi-empirical - from interpolation, extrapolation or another semi-empirical method.

Photos

Photo of the element: Potassium (K) — © images-of-elements.com (CC BY 3.0)

Isotopes (25)

A	N	Half-life	Decay
K-31	12	< 0.01 ns	3p
K-35	16	178 ms	EC/β⁺
K-36	17	341 ms	EC/β⁺
K-37	18	1.2365 s	EC/β⁺
K-38	19	7.645 min	EC/β⁺
K-39	20	stable	-
K-40	21	1.248×10⁹ a	β⁻
K-41	22	stable	-
K-42	23	12.359 h	β⁻
K-43	24	22.28 h	β⁻
K-44	25	22.13 min	β⁻
K-45	26	17.8 min	β⁻
K-46	27	96.31 s	β⁻
K-47	28	17.38 s	β⁻
K-48	29	6.8 s	β⁻
K-49	30	1.26 s	β⁻
K-50	31	472 ms	β⁻
K-51	32	365 ms	β⁻
K-52	33	110 ms	β⁻
K-53	34	30 ms	β⁻
K-54	35	10 ms	β⁻
K-55	36	unknown	β⁻
K-56	37	unknown	β⁻
K-57	38	unknown	β⁻
K-59	40	unknown	β⁻

Data sources

PubChem Periodic Table API - Retrieved on 29 June 2026 - licence: CC0
IUPAC CIAAW - Abridged Standard Atomic Weights 2024 - Table version: 2024 - licence: Public domain
NNDC Wallet Cards 2023 - Data as of 15 October 2024 - licence: Public domain
Images of Elements - Retrieved on 29 June 2026 - licence: CC BY 3.0

More about the data sources