Selenium
Selenium is a chemical element with symbol Se and atomic number 34. It is a nonmetal with properties that are intermediate between those of its periodic table column-adjacent chalcogen elements sulfur and tellurium. It rarely occurs in its elemental state in nature, or as pure ore compounds.
At a glance
- Atomic number
- 34
- Atomic mass
- 78.9718 u
- Phase
- Solid
- Density
- 4.81 g/cm³
- Melting point
- 494 K
- Boiling point
- 958 K
- First ionization energy
- 941 kJ/mol
- Electronegativity
- 2.55
- Electron affinity
- 194.9587 kJ/mol
- Molar heat capacity
- 25.363 J/(mol·K)
- Period
- 4
- Appearance
- black, red, and gray (not pictured) allotropes
Conditions and isotopes affect measured properties. Some properties of short-lived elements are predicted. Electronegativity uses the Pauling scale.
Selenium’s orbitals use a hydrogen-like approximation. Every orbital of each visible subshell is shown: in Shapes as the surface holding half its probability, with light and dark shades for opposite signs of the wavefunction; in Cloud as dots where its electrons are likely to be. Sizes are stepped by shell, like the Shell model.
18 is the [Ar] core total across three shells.
Preparing the orbital view…
34 protons46 neutrons
Selenium-80: its most common form in nature, or its longest-lived if it has none. The nucleus is drawn far too big: it’s about 100,000 times smaller than the atom.
Turn up the heat
A step of 1 K is the same size as 1 °C, so the two scales are always 273.15 apart. Fahrenheit is different: its degrees are smaller and it starts elsewhere, so water freezes at 273.15 K, 0 °C and 32 °F.25 °C · 77 °FSolid
At this temperature, selenium is a solid.
At normal atmospheric pressure (1 atm). The temperature scale is logarithmic. The particle box is schematic: real samples hold vastly more atoms, and spacing and speeds are not to scale.
Pull the electrons off
SeNeutral atom · no charge
A neutral selenium atom. Removing its first electron takes 941 kJ/mol.
Selenium’s outer shell holds 6 electrons. Electron 7 is the first from an inner shell: it sits closer to the nucleus with less shielding, so removing it takes 1.9× as much energy as the one before. Each dashed line on the chart marks another jump, into a shell even closer to the nucleus.
What does this tell us?
- The jumps let you count electrons. Between the dashed lines there are 6, then 18, then 8, then 2 bars. Read backwards, that’s 2, 8, 18, 6: the same electrons per shell as the shell model above. Jumps like these are part of the evidence that shells exist.
- The 6 electrons before the first big jump are selenium’s valence electrons, the ones it uses in chemical reactions. That’s why selenium sits in group 16: every element in the group has the same number of outer electrons, so they react in similar ways.
- Removing electrons from selenium is expensive from the start (941 kJ/mol for the first). So in reactions selenium tends to gain or share electrons rather than lose them.
- Within a shell the steps rise gently. Each electron removed leaves fewer electrons pushing the others away, while the nucleus pulls just as hard, so the next one is held a little more tightly.
Show the values as a table
| Electron | Shell | Energy (kJ/mol) | × previous | Value |
|---|---|---|---|---|
| 1 | 4 | 941 | – | Measured |
| 2 | 4 | 2,045 | 2.2 | Measured |
| 3 | 4 | 3,058 | 1.5 | Measured |
| 4 | 4 | 4,144 | 1.4 | Measured |
| 5 | 4 | 6,590 | 1.6 | Estimated |
| 6 | 4 | 7,895 | 1.2 | Measured |
| 7 | 3 | 14,987 | 1.9 | Measured |
| 8 | 3 | 17,753 | 1.2 | Calculated |
| 9 | 3 | 21,130 | 1.2 | Calculated |
| 10 | 3 | 24,604 | 1.2 | Calculated |
| 11 | 3 | 28,077 | 1.1 | Calculated |
| 12 | 3 | 33,085 | 1.2 | Calculated |
| 13 | 3 | 36,954 | 1.1 | Calculated |
| 14 | 3 | 41,103 | 1.1 | Calculated |
| 15 | 3 | 45,638 | 1.1 | Calculated |
| 16 | 3 | 49,883 | 1.1 | Calculated |
| 17 | 3 | 62,764 | 1.3 | Estimated |
| 18 | 3 | 66,903 | 1.1 | Estimated |
| 19 | 3 | 71,380 | 1.1 | Estimated |
| 20 | 3 | 76,995 | 1.1 | Estimated |
| 21 | 3 | 81,607 | 1.1 | Estimated |
| 22 | 3 | 85,582 | 1 | Estimated |
| 23 | 3 | 95,482 | 1.1 | Estimated |
| 24 | 3 | 99,994 | 1 | Estimated |
| 25 | 2 | 245,140 | 2.5 | Estimated |
| 26 | 2 | 258,002 | 1.1 | Estimated |
| 27 | 2 | 272,089 | 1.1 | Estimated |
| 28 | 2 | 285,983 | 1.1 | Estimated |
| 29 | 2 | 303,543 | 1.1 | Estimated |
| 30 | 2 | 318,575 | 1 | Estimated |
| 31 | 2 | 338,374 | 1.1 | Estimated |
| 32 | 2 | 350,871 | 1 | Calculated |
| 33 | 1 | 1,482,738 | 4.2 | Calculated |
| 34 | 1 | 1,540,688 | 1 | Calculated |
Successive ionization energies of gas-phase atoms and ions from the NIST Atomic Spectra Database. Solid bars are measured; hatched bars were estimated from nearby measurements or calculated from quantum theory, as is usual for highly charged ions. Electrons are taken from the outermost shell first, a simplification: electrons in different subshells of the same shell also differ in energy. On the log scale each gridline is 10 times the one below.
How does it compare?
Density
4.81 g/cm³33rd lowest of 99
Melting point
494 K24th lowest of 96
Boiling point
958 K20th lowest of 97
Electronegativity
2.5511th highest of 96
First ionization energy
941 kJ/mol19th highest of 99
Elements 1–99 from the reference data; heavier elements are left out because only a few atoms have ever been made, so their bulk properties are predictions. First ionization energies come from NIST. Density uses a logarithmic scale, because gases are around a thousand times less dense than solids; gas densities are at 0 °C and 1 atm. Melting and boiling points are at normal pressure. Everyday temperatures and light sources in the pictures are approximate. Hover over a dot to see the element, or click it to compare.
Keep exploring
Explore the stories, imagery, and reference data in more depth. Properties with no value in the reference are marked unavailable.
- SeleniumRoyal Society of Chemistry
- SeleniumWikipedia
- Quantum numbers and electron configurationsPurdue
- Hydrogenic wavefunctionsNIST
- Ionization energiesNIST Atomic Spectra Database