Selenium was discovered by Swedish chemists Jöns Jacob Berzelius and Johan Gottlieb Gahn in 1817 after analyzing an red precipitate that was contaminating the sulfuric acid (H2SO4) being produced at a chemistry plant in Sweden.
Berzelius named the new element after the Moon.
Atomic number: 34
Atomic weight: 78.96
Group, period, block: 16, 4, p
Color: black and red allotropes
Electron configuration: 3d10 4s2 4p4
Density: (gray) 4.81 g/cm-3
Melting point: 494 K,221 °C,430 °F
Boiling point: 958 K,685 °C,1265 °F
Oxidation states: 6, 4, 2, 1, -2
Electronegativity: 2.55 (Pauling scale)
Ionization energies: 1st: 941.0 kJ·mol-1
Covalent radius: 120±4 pm
Van der Waals radius: 190 pm
There are six naturally occurring isotopes of selenium exist, 74Se, 76Se, 77Se, 78Se, 80Se, and 82Se.