Element: Argon

General data

Element name	Argon
Chemical symbol	Ar
Atomic number Z	18
Year discovered	1894
Standard state (25°C)	Gas
Category	Noble gas
Block	p
Atomic mass	39.95
CPK color	#80D1E3

Physical properties

Melting point	83.8 K (-189.3 °C)
Boiling point	87.3 K (-185.8 °C)
Density	0.0017837 g/cm³

Energy properties

First ionization energy	15.76 eV
Electron affinity	no data

Atomic structure

Protons	18 (Atomic number)
Neutrons (Ar-40, most abundant isotope)	22
Electrons	18
Electron configuration	[Ne] 3s² 3p⁶
Oxidation states	no data
Electronegativity (Pauling)	no data
Atomic radius (van der Waals)	188 pm

Ionization energies - NIST ASD (18 states)

State	Charge	Ioniz. energy (eV)	Type
Ar I	0+	15.7596119 ± 0.0000005	measured
Ar II	1+	27.62967 ± 0.00012	measured
Ar III	2+	40.735 ± 0.012	semi-empirical
Ar IV	3+	59.58 ± 0.18	semi-empirical
Ar V	4+	74.84 ± 0.17	semi-empirical
Ar VI	5+	91.29 ± 0.01	measured
Ar VII	6+	124.41 ± 0.06	semi-empirical
Ar VIII	7+	143.4567 ± 0.0012	semi-empirical
Ar IX	8+	422.6 ± 0.06	semi-empirical
Ar X	9+	479.76 ± 0.12	semi-empirical
Ar XI	10+	540.4 ± 0.6	semi-empirical
Ar XII	11+	619 ± 0.4	semi-empirical
Ar XIII	12+	685.5 ± 0.3	semi-empirical
Ar XIV	13+	755.13 ± 0.22	semi-empirical
Ar XV	14+	855.5 ± 0.3	semi-empirical
Ar XVI	15+	918.375 ± 0.007	calculated
Ar XVII	16+	4120.66559 ± 0.00022	calculated
Ar XVIII	17+	4426.22407 ± 0.00004	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: Argon (Ar) — © images-of-elements.com (CC BY 3.0)

Isotopes (26)

A	N	Half-life	Decay
Ar-29	11	unknown	2p
Ar-30	12	< 10 ps	2p
Ar-31	13	15.0 ms	EC/β⁺
Ar-32	14	98 ms	EC/β⁺
Ar-33	15	173 ms	EC/β⁺
Ar-34	16	846.5 ms	EC/β⁺
Ar-35	17	1.7754 s	EC/β⁺
Ar-36	18	stable	-
Ar-37	19	35.01 d	EC
Ar-38	20	stable	-
Ar-39	21	268 a	β⁻
Ar-40	22	stable	-
Ar-41	23	109.61 min	β⁻
Ar-42	24	32.9 a	β⁻
Ar-43	25	5.37 min	β⁻
Ar-44	26	11.87 min	β⁻
Ar-45	27	21.48 s	β⁻
Ar-46	28	8.4 s	β⁻
Ar-47	29	1.23 s	β⁻
Ar-48	30	416 ms	β⁻
Ar-49	31	236 ms	β⁻
Ar-50	32	105 ms	β⁻
Ar-51	33	unknown	β⁻
Ar-52	34	unknown	β⁻
Ar-53	35	unknown	β⁻
Ar-54	36	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