Selenium (Se)
Selenium is the 34th element, a reactive nonmetal in the p-block. Its standard atomic weight is 78.96.
Selenium
78.96
What the table records
- Atomic number
- 34
- Standard atomic weight
- 78.96
- Group
- 16
- Period
- 4
- Block
- p-block
- Category
- Reactive nonmetal
- [Ar] 3d10 4s2 4p4
- Valence electrons
- 6
- Typical valence
- 2
- Van der Waals radius
- 1.9 Å
- Covalent radius
- 1.2 Å
Molecules containing selenium
The lightest named structures in this library that contain Se.
Oxyselenide
OSe · 94.96 g/mol
Selenium Dioxide
O₂Se · 110.96 g/mol
Benzeneselenol
C₆H₆Se · 157.07 g/mol
3,5-Dimethylphenyl selenol
C₈H₁₀Se · 185.13 g/mol
Benzeneseleninic acid
C₆H₆O₂Se · 189.07 g/mol
Chloroselenobenzene
C₆H₅ClSe · 191.52 g/mol
Ethanol, 2-(phenylseleno)-
C₈H₁₀OSe · 201.13 g/mol
Methyl 5-methylselenophene-2-carboxylate
C₇H₈O₂Se · 203.1 g/mol
2-Bromoselenophene
C₄H₃BrSe · 209.93 g/mol
3-Amino-4,5,6,7-tetrahydrobenzo[1,2-b]selenophene-2-carbonitrile
C₉H₁₀N₂Se · 225.15 g/mol
Benzene, 1,1'-selenobis-
C₁₂H₁₀Se · 233.17 g/mol
3-Amino-4,5,6,7-tetrahydrobenzo[1,2-b]selenophene-2-carboxamide
C₉H₁₂N₂OSe · 243.17 g/mol
How to read these numbers
The standard atomic weight is an average over the isotopes as they occur naturally, which is why it is rarely a whole number. It is the figure a molar mass calculation uses. Work in mass spectrometry wants the monoisotopic mass of a single isotope instead, which is a different number.
The two radii are not the same measurement. The covalent radius is half the distance between two bonded atoms of the element; the van der Waals radius is how close a non-bonded atom gets before repulsion takes over, and is always the larger. Space-filling models are drawn with the second, ball-and-stick models with the first — which is why the same molecule looks so different in the two.