Chemical element · reference page

34Se

Selenium

Selenium is element 34 (Se), located in period 4, group 16 and the p block. At room temperature its reference phase is solid. Its electron configuration is [Ar] 3d10 4s2 4p4.

Atomic number34

Standard atomic weight78.971(8)

Group16

Period4

Blockp

Electron configuration[Ar] 3d10 4s2 4p4

Electronic structure

What controls its chemistry

Group 16 atoms have six valence electrons. Bonding often involves gaining or sharing two electrons, while the heavier members can access a wider range of oxidation states.

For selenium, this outer-shell structure helps explain the importance of oxides, sulfides, oxoanions and covalent compounds across the chalcogen family.

Chemical forms

Compounds, ions and materials

Hydrides, oxides, sulfides or selenides, halides and oxoanions are important across group 16. The −2 oxidation state is common for the lighter chalcogens, while positive oxidation states become increasingly accessible in heavier compounds.

Physical behaviour

State and phase changes

The reference phase at room temperature is solid. The listed density is 4.81 g/cm³. The melting point is 494.2 K (221.1 °C). The boiling point is 958 K (684.9 °C). Pressure, purity and crystal structure can shift measured physical properties.

Occurrence and applications

Where selenium is found and used

In practice, selenium is encountered in special glasses, electronics and biology.

In practice

Examples of real uses

Special glasses

Selenium is added to glass to modify refractive index, colour, thermal expansion, radiation response or chemical durability.

Electronics

Selenium is used in electronic components or materials where conductivity, semiconducting behaviour, optical response or chemical stability is useful.

Biology

Selenium ions or compounds participate in biological structures, enzymes, signalling or metabolism.

Periodic position

What its neighbours tell us

Selenium is in period 4 and group 16. In the neighbourhood shown here, it lies between arsenic and bromine in the same period; sulfur is above it and tellurium below it in the same group. These positions make it possible to compare atomic size, ionisation energy and bonding behaviour with nearby elements.

Reference data

Physical and atomic properties

Classificationpolyatomic nonmetal

Phase at room temperatureSolid

Electron shells2 · 8 · 18 · 6

Density4.81 g/cm³

Melting point494.2 K (221.1 °C)

Boiling point958 K (684.9 °C)

Pauling electronegativity2.55

First ionisation energy941 kJ/mol

Electron affinity195 kJ/mol

Values describe the element or neutral atom where applicable. Physical data can depend on allotrope, pressure and measurement conditions.

History

Discovery and identification

The discovery of selenium is associated with Jöns Jakob Berzelius. Recognition of the element, isolation of a pure sample and assignment of its modern atomic number did not necessarily occur at the same time.

Isotopes and atomic weight

How isotope composition changes the numbers

The standard atomic weight is 78.971(8). This value refers to the isotopic composition of natural terrestrial material, not to the mass of one specific atom.

Selenium has at least one stable isotope. Different isotopes have the same number of protons and therefore the same element identity, but different neutron numbers and atomic masses.

Safety

Handling and exposure

The hazards associated with selenium depend on chemical form, dose and route of exposure. Pure selenium, its ions and its compounds can have very different biological and environmental effects, so safety data should be checked for the actual substance being handled.