Selenium (Se)

Selenium's electrical resistance drops when light falls on it, a property discovered in 1873 that led directly to the first light meters, the first photocells and eventually to xerography — every photocopier before the 1980s worked on a selenium drum. The gap between a necessary dose and a toxic one is unusually narrow: about 55 micrograms a day is required and roughly 400 is the safe upper limit. In parts of China with selenium-poor soil, deficiency causes a heart disease called Keshan disease, while a few selenium-rich areas produce hair loss and brittle nails from excess.

Properties

Atomic number

34

Symbol

Se

Standard atomic weight

78.971 u

Category

Nonmetal

Group

16

Period

4

Block

p-block

Electron configuration

[Ar] 4s² 3d¹⁰ 4p⁴

Electrons per shell

2, 8, 18, 6

Melting point

493.65 K — 220.5 °C — 428.9 °F

Boiling point

958 K — 684.9 °C — 1264.7 °F

Density

4.809 g/cm³

Electronegativity

2.55 (Pauling)

Oxidation states

+6, +4, -2

Discovered

1817 by Jöns Jacob Berzelius

Electron configuration

Full

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁴

Noble gas shorthand

[Ar] 4s² 3d¹⁰ 4p⁴

Shells

K: 2 · L: 8 · M: 18 · N: 6

What it is used for

• Decolourising the green tint of iron in bottle glass
• Selenium sulfide in anti-dandruff shampoo
• Photoreceptor drums in older photocopiers and laser printers
• An essential trace nutrient, and an additive in livestock feed
• Pigments — the red in some traffic lights and warning glass

Hazards

• The margin between the required intake and toxicity is small, roughly a factor of seven
• Hydrogen selenide is highly toxic and detectable by smell only above dangerous levels

Neighbours in the table

Group 16

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Selenium sits in group 16 with oxygen, sulfur, tellurium, polonium, livermorium.