Selenium at a glance
- Atomic number
- 34
- Symbol
- Se
- Atomic mass
- 78.971 u
- Category
- Reactive nonmetal
- Group / period
- 16 / 4
- Electron configuration
- [Ar] 3d 10 4s 2 4p 4
- State at 25 °C
- Solid
- Melting point
- 220.9 °C
- Boiling point
- 684.9 °C
- Density
- 4.809 g/cm³
- Electronegativity
- 2.55
- 1st ionization energy
- 941 kJ/mol
- Oxidation states
- −2, +4, +6
- Discovered
- 1817
Atomic structure
A neutral selenium atom has 34 protons and 34 electrons, and its most common isotope carries about 46 neutrons. One atom weighs about 1.31 × 10⁻²⁵ kg, so a mole of selenium (6.022 × 10²³ atoms) weighs 78.971 g. That nucleus holds 1.35 neutrons for every proton. The electrons fill 4 shells (K 2, L 8, M 18, N 6), which gives the configuration [Ar] 3d 10 4s 2 4p 4. With 6 electrons in its outer shell, it tends to gain or share electrons to complete its outer shell. It has 6 stable isotopes: Se-74, Se-76, Se-77, Se-78, Se-80 and Se-82. Its most common isotope, Se-80, has the same mass number as Kr-80: isobars, nuclei of different elements with 80 nucleons each.
Physical properties
At room temperature (25 °C) selenium is a solid. It melts at 220.9 °C and boils at 684.9 °C, so it stays liquid across a 464-degree range. It stays solid in boiling water but would melt in a hot kitchen oven. Its density is 4.809 g/cm³, 4.81 times as dense as water. Its atoms have a radius of about 115 pm (Arsenic next door is 115 pm). In the solid, its atoms pack into a hexagonal lattice.
Chemical behaviour
Its electronegativity is 2.55 on the Pauling scale. Pulling off its outermost electron takes 941 kJ/mol, less than Sulfur above it (1000 kJ/mol). A bond between selenium and chlorine would be polar covalent, with an electronegativity difference of 0.61. It releases 195 kJ/mol when it gains an extra electron. Its common oxidation states are −2, +4, +6, so in compounds you will usually meet it as Se²⁻, Se⁴⁺ and Se⁶⁺. Group 16 also holds Oxygen, Sulfur, Tellurium, Polonium and Livermorium, which share its outer-shell pattern. Its Lewis symbol is Se surrounded by 6 dots. Handle with care: as a pure element it is toxic.
Where it comes from
It makes up 0.05 mg/kg of Earth's crust, the 69th most abundant element there. In the human body it accounts for about 0.00002 % of body mass. It was first identified in 1817, making it the 49th element to be discovered (counting the 11 known since antiquity together).