Selenium is named after Selene, the Greek goddess of the moon. This element is a semimetal and therefore has the properties of both metals and nonmetals. Selenium has two main pure forms: gray selenium, which is a hard substance, and red selenium, which is a soft powder. The most common use of selenium is as an ingredient that provides color in glass and ceramics. Selenium is sensitive to light, so it is used in solar cells that convert sunlight into electricity. It is also used in photocopiers.
Summary of properties (Se)
| Atomic weight | 78.971(8) |
| Discoverer (year) | Berzelius, Jöns Jacob (1817) |
| Natural form | non-metal solid (hexagonal) |
| Electron configuration | [Ar] 3d10 4s2 4p4 |
| Melting point (ºC) | 217 |
| Boiling point (ºC) | 685 |
| Abundance in earth's crust (ppm) | 0.05 |
| Isotope (abundance) | 74Se (0.86), 76Se (9.23), 77Se (7.60), 78Se (23.69), 80Se (49.80), 82Se (8.82) |
| Density g/cm3 | 4.79 |
| Van der Waals radius (pm) | 190 |
| Covalent radius (pm) | 118 |
| Electronegativity (Pauling) | 2.96 |
| Vaporization enthalpy (kJ/mol) | 95.48 |
| Enthalpy of fusion (kJ/mol) | 6.69 |
| Specific heat capacity (J/g·K) at 25ºC and 1 atm | 0.32 |
| Thermal conductivity (W/cm·K) at 25ºC and 1 atm | 0.200 |
| Oxidation state | +6, +4, -2 |
| Electron affinity (eV) | 2.02 |
| 1st Ionization potential (eV) | 9.7524 |
