Selenium (Se)
Selenium (Se) is a reactive nonmetal with atomic number 34 and a standard atomic weight of 78.971. At room temperature it is a solid that melts at 221 °C and boils at 685 °C.
Selenium key facts
Selenium at a glance
Identity
| Symbol | Se |
|---|---|
| Atomic number (Z) | 34 |
| Atomic weight | 78.971 ± 0.008 (CIAAW 2024) |
| Category | Reactive nonmetal |
| Group | 16 — chalcogens (oxygen group) |
| Period | 4 |
| Block | p-block |
| CAS number | 7782-49-2 |
Atomic properties
| Electron configuration | [Ar] 3d10 4s2 4p4 Full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p4 |
|---|---|
| Electrons per shell | 2, 8, 18, 6 |
| Oxidation states | -2, +2, +4, +6 Less common: -1, 0, +1, +3, +5 |
| Electronegativity (Pauling) | 2.55 |
| First ionisation energy | 9.752 eV (941 kJ/mol) 2nd: 21.196 eV · 3rd: 31.697 eV |
| Electron affinity | 2.021 eV |
| Atomic radius (empirical) | 115 pm |
| Covalent radius | 116 pm |
| Van der Waals radius | 190 pm |
Physical properties
| State at 25 °C | Solid |
|---|---|
| Melting point | 220.9 °C · 429.5 °F · 494 K |
| Boiling point | 685 °C · 1,265 °F · 958.15 K |
| Density | 4.809 g/cm³ |
| Specific heat capacity | 0.321 J/(g·K) |
| Thermal conductivity | 0.52 W/(m·K) |
| Crystal structure | Hexagonal |
Isotopes & abundance
| Natural occurrence | Primordial |
|---|---|
| Radioactive | No |
| Natural isotopes (abundance) | 80Se 49.8%, 78Se 23.69%, 76Se 9.23%, 82Se 8.82% (radioactive), 77Se 7.6%, 74Se 0.86% |
| Stable isotopes | 5 |
| Known isotopes | 33 |
| Abundance in Earth's crust | 0.05 mg/kg |
| Abundance in seawater | 0.0002 mg/L |
Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.
Selenium electron configuration
Selenium has 34 electrons. Written in short form, its ground-state configuration is [Ar] 3d10 4s2 4p4 — the electrons of the noble gas argon, plus 3d10 4s2 4p4.Written out in full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p4.
As a member of group 16 it has 6 valence electrons in its outer shell (shell 4), which largely decide its chemistry.
Discovery and name
1817 — Jöns Jacob Berzelius and Johan Gottlieb Gahn · Sweden.
Greek selēnē, the Moon — named as a counterpart to tellurium (Earth).
Known for
An essential trace element. Its conductivity rises in light, which made it useful in photocopiers and light meters; used to decolourise glass.
Isotopes of selenium
Naturally occurring selenium is a mix of 6 isotopes. Each has 34 protons; they differ in the number of neutrons:
| Isotope | Protons | Neutrons | Natural abundance | Stability |
|---|---|---|---|---|
| 80Se | 34 | 46 | 49.8% | stable |
| 78Se | 34 | 44 | 23.69% | stable |
| 76Se | 34 | 42 | 9.23% | stable |
| 82Se | 34 | 48 | 8.82% | radioactive · half-life 87.6 × 10¹⁸ years |
| 77Se | 34 | 43 | 7.6% | stable |
| 74Se | 34 | 40 | 0.86% | stable |
33 isotopes of selenium are known in our data set (half-lives as published in nuclear data tables).
Selenium compared with group 16
The other elements in group 16 (chalcogens (oxygen group)), from top to bottom of the table:
| Element | Z | Atomic weight | Melting point | Boiling point | Density | Electronegativity |
|---|---|---|---|---|---|---|
| Oxygen | 8 | 15.999 | -219 °C | -183 °C | 1.429 g/L | 3.44 |
| Sulfur | 16 | 32.06 | 115 °C | 445 °C | 2.07 g/cm³ | 2.58 |
| Selenium | 34 | 78.971 | 221 °C | 685 °C | 4.809 g/cm³ | 2.55 |
| Tellurium | 52 | 127.60 | 450 °C | 988 °C | 6.232 g/cm³ | 2.1 |
| Polonium | 84 | [209] | 254 °C | 962 °C | 9.2 g/cm³ | 2 |
| Livermorium | 116 | [293] | — | — | 12.9 g/cm³ (pred.) | — |