Selenium

Selenium (Se) is a reactive nonmetal with atomic number 34 and an atomic mass of 78.971. Its electron configuration is [Ar] 3d10 4s2 4p4, which gives it 6 valence electrons.

Selenium key data

Symbol	Se
Atomic number	34
Atomic mass (standard atomic weight)	78.971
Molar mass	78.971 g/mol
Protons	34
Neutrons (most abundant isotope, selenium-80)	46
Electrons (neutral atom)	34
Valence electrons	6
Category	Reactive nonmetal
Period	4
Group	16
Block	p-block
Noble gas notation	[Ar] 3d10 4s2 4p4
Full electron configuration	1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p4
Electrons per shell	2, 8, 18, 6

Atomic mass: IUPAC CIAAW abridged standard atomic weights, 2024. Most abundant isotope: NIST Atomic Weights and Isotopic Compositions.

Measured properties of selenium

From PubChem, the US National Institutes of Health. Public domain.

Melting point	493.65 K (220.5 °C), 79th of 103 by melting point
Boiling point	958 K (684.85 °C), 75th of 93 by boiling point
Density	4.809 g/cm³, 65th of 96 by density
State at room temperature	Solid
Electronegativity	2.55 (Pauling), joint 10th of 95 by electronegativity
Van der Waals radius	190 pm, 71st of 99 by atomic radius
First ionisation energy	9.752 eV (940.9 kJ/mol), 19th of 102 by first ionisation energy
Electron affinity	2.021 eV (195 kJ/mol), 9th of 57 by electron affinity
Common oxidation states	+6, +4, −2
Discovered	1817

Each rank counts down from the highest value, among the elements PubChem’s table gives one for, and links to that property ranked across the whole table. All of them are collected under periodic trends.

Selenium electron configuration

The full electron configuration of selenium is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p4. With the noble-gas core collapsed it is [Ar] 3d10 4s2 4p4, and the outermost subshell is 4p4.

Bohr model of selenium (Se): a nucleus and four shells holding 2, 8, 18 and 6 electrons from the inside out. — Bohr model of selenium (Se). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Orbital box diagram of selenium (Se), 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p4: one box per orbital, filled by Hund’s rule, showing two unpaired electrons, in 4p. — Orbital diagram of selenium (Se). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Why group and block follow from this is set out in how the periodic table maps electron configuration, and the electron configuration of every element is listed on one page.

Mass of one selenium atom

A molar mass is grams per mole, so dividing by the Avogadro constant gives the mass of a single atom, or for an element with more than one natural isotope the average mass of its atoms, and the reciprocal gives how many atoms sit in a gram.

Mass of one atom

one atom = 78.971 / (6.02214076 × 10²³)
= 1.3113 × 10⁻²² g

Atoms in one gram

atoms per gram = 6.02214076 × 10²³ / 78.971
= 7.6258 × 10²¹ atoms

Selenium on the periodic table

Selenium is in period 4, group 16, in the p-block.

The other elements of group 16 are oxygen, sulfur, tellurium, polonium and livermorium.