Periodic Table


Atomic number: 34
Atomic weight: 78.96
Symbol: Se
Group number: 16
Electronic configuration: [Ar].3d10.4s2.4p4


Selenium can be prepared with either an amorphous or crystalline structure. Crystalline monoclinic selenium is deep red; crystalline hexagonal selenium, the most stable variety, is a metallic grey. Elemental selenium is relatively nontoxic and is considered to be an essential trace element. However, hydrogen selenide (H2Se) and other selenium compounds are extremely toxic, and resemble arsenic in their physiological reactions. Hydrogen selenide in a concentration of 1.5 ppm is intolerable to man. Selenium occurs in some soils in amounts sufficient to produce serious effects on animals feeding on plants such as locoweed (an American plant) grown in such soils.

General information

Discoveror: Jöaut;ns Jacob Berzelius
Date discovered: 1817
Discovered at: Sweden
Meaning of name: From the Greek word "selene" meaning "moon"

Physical data

Standard state: solid at 298 K
Colour: grey, metallic lustre
Density of solid at ambient temperature/kg m-3: 4819
Molar volume/cm3: 16.42

Radii /pm

Atomic: 103
Covalent (single bond):
Pauling radius for the ion [Se]-: 232

Valence shell orbital radius maxima (Rmax)
orbital s p d f
radius82.496.824.1no data


Both values are quoted on the Pauling scale.

Pauling: 2.55
Allred Rochow: 2.48

Crystal Structure

structure: monoclinic

The following CrystalMaker image represents the solid state structure. For most elements, the most stable allotrope is illustrated. Try WebElements version 2 for interactive virtual reality and CHIME images.


Temperatures (/K)

melting point: 494
boiling point: 958

Enthalpies /kJ mol-1

fusion: 5.4
vaporization: 95

single bond enthalpies:
Se-F Se-Cl Se-Br Se-I Se-Se
285 192 151 151 332.6

Ionization enthalpies /kJ mol-1

Number Enthalpy


This section gives some data for naturally occurring isotopes.
Nominal mass Accurate mass % natural abundance
74Se73.9224746 (16)0.89 (2)
76Se75.9192120 (16)9.36 (11)
77Se76.9199125 (16)7.63 (6)
78Se77.9173076 (16)23.78 (9)
80Se79.9165196 (19)49.61 (10)
82Se81.9166978 (23)8.73 (6)

Further Information

Copyright 1997 Mark Winter
Department of Chemistry at the University of Sheffield, Sheffield S3 7HF, England.

The current version of this document is at http://www.shef.ac.uk/~chem/web-elements-I/Se.html