Selenium

Named for the moon, needed by your body in tiny amounts, and poisonous in slightly larger ones.

At a glance

Symbol: Se
Atomic number: 34
Average atomic mass: 78.97
Group: 16
Period: 4
Family: nonmetal
Electron configuration: [Ar] 3d¹⁰ 4s² 4p⁴
Valence electrons: 6
Electronegativity: 2.55
Common ions: Se²⁻ (selenide); +4 and +6 with oxygen

On the table

Period 4, group 16. The other nonmetals are tinted, and every cell opens its own article.

Electrons and bonding

Selenium has 34 electrons, one for every proton, so the atom carries no charge. Below are the same 34 electrons written two ways.

Simple form: electrons per shell

shell 12shell 28shell 318shell 46

Counting outward from the nucleus: 2, 8, 18, 6. The outer shell is picked out.

Orbital form: electrons per subshell

1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁴

Short form: [Ar] 3d¹⁰ 4s² 4p⁴, where [Ar] stands for the 18 electrons argon already has.

That list runs by shell. In the order the subshells fill it reads [Ar] 4s² 3d¹⁰ 4p⁴, because a subshell fills by its energy, not by its shell number.

3d¹⁰↑↓↑↓↑↓↑↓↑↓

4s²↑↓

4p⁴↑↓↑↑

The boxes hold the electrons beyond [Ar]. Each box is one orbital and takes two electrons at most, spinning opposite ways. They spread out singly before they pair up.

How to read the two

The simple form counts electrons shell by shell. The orbital form says which subshell each one sits in: s holds 2, p holds 6, d holds 10 and f holds 14. Shell 3 here is 3s² 3p⁶ 3d¹⁰, and 2 + 6 + 10 is 18.

Selenium sits in period 4 and has 4 shells. The period number counts the shells.

What that means for bonding

The outer shell holds 6 electrons. Those are the ones selenium can lose, gain or share, so they are the ones that decide what it bonds with. For a main-group element the group number gives that count: group 16 means 6.

A nonmetal: it gains electrons to form negative ions with metals and shares them in covalent bonds with other nonmetals.

Properties

State at 25 °C: solid
What it looks like: gray metallic crystals; it also has a red powdery form and a black glassy form
Melting point: 220.5 °C
Boiling point: 685 °C
Density: 4.81 g/cm³

Where it is found

Rare, found with sulfur in copper and other sulfide ores, and nearly all of it is recovered during copper refining. Brazil nuts are especially rich in it.

Discovery and name

Jöns Jacob Berzelius and Johan Gottlieb Gahn found it in 1817 in the red sludge of a sulfuric acid plant. Because it resembled tellurium, named for the Earth, Berzelius named it from the Greek selene, moon.

What it is used for

Glassmaking, to cancel the green tint iron gives glass or to make it red; producing manganese metal; thin-film solar cells; and anti-dandruff shampoo, which contains selenium sulfide. Its conductivity rises in light, which made it the heart of early photocopiers and light meters. It is also added to animal feed.

In compounds

SeS₂selenium disulfide, in dandruff shampooSeO₂selenium dioxideH₂Sehydrogen selenide, very toxicCdSecadmium selenide, quantum dotsNa₂SeO₃sodium selenite, a supplement

Handle with care

An essential nutrient in micrograms and toxic in milligrams. Hydrogen selenide gas is especially dangerous.