Selenium

34
Se
Group
16
Period
4
Block
p
Protons
Electrons
Neutrons
34
34
45
General Properties
Atomic Number
34
Atomic Weight
78.96
Mass Number
79
Category
Other nonmetals
Colour
Gray
Radioactive
No
From the Greek word Selene, moon
Crystal Structure
Simple Monoclinic
History
Selenium was first observed in about the year 1300 by the alchemist Arnold of Villanova.

Selenium was discovered in 1817 by Jöns Jacob Berzelius and Johan Gottlieb Gahn who noted the similarity of the new element to the previously-known tellurium.

In 1873, Willoughby Smith found that the electrical resistance of grey selenium was dependent on the ambient light.
Electrons per shell
2, 8, 18, 6
Electron Configuration
[Ar] 3d10 4s2 4p4
Se
Selenium deficiency in animals can lead to slow growth
Physical Properties
Phase
Solid
Density
4.809 g/cm3
Melting Point
494.15 K | 221 °C | 429.8 °F
Boiling Point
958.15 K | 685 °C | 1265 °F
Heat of Fusion
5.4 kJ/mol
Heat of Vaporization
26 kJ/mol
Specific Heat Capacity
0.321 J/g·K
Abundance in Earth's crust
5×10-6%
Abundance in Universe
3×10-6%
Ultrapure
Image Credits: Images-of-elements
Ultrapure black, amorphous selenium
CAS Number
7782-49-2
PubChem CID Number
6326970
Atomic Properties
Atomic Radius
120 pm
Covalent Radius
120 pm
Electronegativity
2.55 (Pauling scale)
Ionization Potential
9.7524 eV
Atomic Volume
16.45 cm3/mol
Thermal Conductivity
0.0204 W/cm·K
Oxidation States
-2, 2, 4, 6
Applications
Selenium is used in the glass industry to decolourize glass and to make red-coloured glasses and enamels.

It is used as a catalyst in many chemical reactions.

It is also used as a photographic toner, and as an additive to stainless steel.

Selenium sulphide is used in anti-dandruff shampoos.
Many of selenium's compounds, such as selenates and selenites, are highly toxic
Isotopes
Stable Isotopes
74Se, 76Se, 77Se, 78Se, 80Se
Unstable Isotopes
65Se, 66Se, 67Se, 68Se, 69Se, 70Se, 71Se, 72Se, 73Se, 75Se, 79Se, 81Se, 82Se, 83Se, 84Se, 85Se, 86Se, 87Se, 88Se, 89Se, 90Se, 91Se, 92Se, 93Se, 94Se