34 Se

Selenium (Se)

Element 34 · Reactive nonmetal · Solid at 25 °C · Group 16, Period 4

Selenium's conductivity changes with light, which made photocopiers and early photocells possible. In trace amounts it is an essential nutrient.

On the periodic table, selenium sits in group 16 — the chalcogens (oxygen group) — in period 4. It is one of 7 elements classified as a reactive nonmetal, and at room temperature (25 °C) it is a solid.

The reactive nonmetals are a diverse family that includes hydrogen, carbon, nitrogen, oxygen, phosphorus, sulfur, and selenium. They gain or share electrons in chemical reactions, forming the covalent bonds that hold molecules together. Most of the atoms in living things — including virtually all of you — belong to this category.

Properties of selenium

Properties of Selenium
Atomic number (Z)	34
Symbol	Se
Atomic mass	78.971
Category	Reactive nonmetal
Group	16
Period	4
Block	p-block
Metal / nonmetal	Nonmetal
State at 25 °C	Solid
Electron configuration	[Ar] 3d10 4s2 4p4
Electronegativity (Pauling)	2.55
Melting point	221 °C (494.15 K / 429.8 °F)
Boiling point	685 °C (958.15 K / 1,265 °F)
Density	4.809 g/cm³

Position in the periodic table

Selenium shares group 16 — the chalcogens (oxygen group) with Oxygen (O), Sulfur (S), Tellurium (Te), Polonium (Po), Livermorium (Lv). Elements in the same group have the same number of outer electrons, so they tend to behave alike chemically — comparing them is one of the fastest ways to see periodic trends in action.

Physical properties

Selenium melts at 221 °C (494.15 K / 429.8 °F) and boils at 685 °C (958.15 K / 1,265 °F), giving it a liquid range of about 464 °C at standard pressure.

Its density is 4.809 g/cm³.