Element: Selenium

General data

Element name	Selenium
Chemical symbol	Se
Atomic number Z	34
Year discovered	1817
Standard state (25°C)	Solid
Category	Nonmetal
Block	p
Atomic mass	78.971
CPK color	#FFA100

Physical properties

Melting point	493.65 K (220.50 °C)
Boiling point	958 K (685 °C)
Density	4.809 g/cm³

Energy properties

First ionization energy	9.752 eV
Electron affinity	2.021 eV

Atomic structure

Protons	34 (Atomic number)
Neutrons (Se-80, most abundant isotope)	46
Electrons	34
Electron configuration	[Ar] 4s² 3d¹⁰ 4p⁴
Oxidation states	+6, +4, -2
Electronegativity (Pauling)	2.55
Atomic radius (van der Waals)	190 pm

Ionization energies - NIST ASD (34 states)

State	Charge	Ioniz. energy (eV)	Type
Se I	0+	9.752368 ± 0.000006	measured
Se II	1+	21.196 ± 0.01	measured
Se III	2+	31.697 ± 0.019	measured
Se IV	3+	42.947 ± 0.003	measured
Se V	4+	68.3 ± 0.1	semi-empirical
Se VI	5+	81.83 ± 0.03	measured
Se VII	6+	155.327 ± 0.012	measured
Se VIII	7+	184 ± 2.1	calculated
Se IX	8+	219 ± 2.2	calculated
Se X	9+	255 ± 2.2	calculated
Se XI	10+	291 ± 2.4	calculated
Se XII	11+	342.9 ± 2.5	calculated
Se XIII	12+	383 ± 3	calculated
Se XIV	13+	426 ± 3	calculated
Se XV	14+	473 ± 3	calculated
Se XVI	15+	517 ± 3	calculated
Se XVII	16+	650.5 ± 2.1	semi-empirical
Se XVIII	17+	693.4 ± 0.7	semi-empirical
Se XIX	18+	739.8 ± 1.1	semi-empirical
Se XX	19+	798 ± 3	semi-empirical
Se XXI	20+	845.8 ± 2	semi-empirical
Se XXII	21+	887 ± 4	semi-empirical
Se XXIII	22+	989.6 ± 1.3	semi-empirical
Se XXIV	23+	1036.36 ± 0.03	semi-empirical
Se XXV	24+	2540.7 ± 1.4	semi-empirical
Se XXVI	25+	2674 ± 5	semi-empirical
Se XXVII	26+	2820 ± 6	semi-empirical
Se XXVIII	27+	2964 ± 22	semi-empirical
Se XXIX	28+	3146 ± 3	semi-empirical
Se XXX	29+	3301.8 ± 2.1	semi-empirical
Se XXXI	30+	3507 ± 3	semi-empirical
Se XXXII	31+	3636.526 ± 0.007	calculated
Se XXXIII	32+	15367.493 ± 0.003	calculated
Se XXXIV	33+	15968.1075 ± 0.0012	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: Selenium (Se) — © images-of-elements.com (CC BY 3.0)

Isotopes (33)

A	N	Half-life	Decay
Se-63	29	13 ms	EC/β⁺
Se-64	30	22.6 ms	EC/β⁺
Se-65	31	34.2 ms	EC/β⁺
Se-66	32	51 ms	EC/β⁺
Se-67	33	133 ms	EC/β⁺
Se-68	34	35.5 s	EC/β⁺
Se-69	35	27.4 s	EC/β⁺
Se-70	36	41.1 min	EC/β⁺
Se-71	37	4.78 min	EC/β⁺
Se-72	38	8.40 d	EC
Se-73	39	7.18 h	EC/β⁺
Se-74	40	stable	-
Se-75	41	119.779 d	EC
Se-76	42	stable	-
Se-77	43	stable	-
Se-78	44	stable	-
Se-79	45	3.3×10⁵ a	β⁻
Se-80	46	stable	-
Se-81	47	18.5 min	β⁻
Se-82	48	8.8×10¹⁹ a	2β⁻
Se-83	49	22.25 min	β⁻
Se-84	50	3.3 min	β⁻
Se-85	51	32.9 s	β⁻
Se-86	52	14.3 s	β⁻
Se-87	53	5.51 s	β⁻
Se-88	54	1.51 s	β⁻
Se-89	55	0.41 s	β⁻
Se-90	56	200 ms	β⁻
Se-91	57	270 ms	β⁻
Se-92	58	unknown	β⁻
Se-93	59	unknown	β⁻
Se-94	60	unknown	β⁻
Se-95	61	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