Selenium · element 34
Named after the Moon, essential in tiny doses, toxic in slightly larger ones, and the reason photocopiers worked
Key facts
Selenium walks a narrow line. Your body needs a few dozen micrograms a day to build enzymes that protect cells and activate thyroid hormone. Take ten times that amount regularly and your hair falls out and your breath smells of garlic. Soils in some parts of the world have too little, others have so much that grazing animals are poisoned.
It is a non-metal in group 16, directly under sulfur, and comes in several forms: a grey metallic-looking solid, a red powder and a black glassy mass. The grey form conducts electricity much better in light than in darkness, and that single property made it the heart of early photocopiers and light meters.
Where it is found
Selenium is rare in the crust and seldom forms minerals of its own. It substitutes for sulfur in sulfide ores, particularly copper ores. Nearly all commercial selenium is recovered from the sludge that collects at the bottom of the tanks when copper is purified by electrolysis. China, Japan and Germany are major refiners.
In soils its distribution is uneven. Parts of the American Great Plains and the Enshi district of China are selenium-rich, while a belt across parts of China and much of Finland and New Zealand is deficient. Brazil nuts concentrate it strongly. Finland has added selenium to fertilisers since 1984 to raise levels in the national diet.
How it was discovered
In 1817 Jöns Jacob Berzelius and Johan Gottlieb Gahn were part-owners of a sulfuric acid works at Gripsholm in Sweden. They noticed a reddish sediment building up in the lead chambers where the acid was made. It smelled of horseradish when heated, which pointed to tellurium, but the chemistry did not fit.
Berzelius showed it was a new element with properties between sulfur and tellurium. Since tellurium had been named after the Earth (tellus in Latin), he named its lighter relative after the Moon, selene in Greek. It is one of the few elements discovered as an industrial waste product.
What we use it for
The largest single use is surprisingly obscure: selenium dioxide is added during the electrolytic production of manganese metal, which improves the process efficiency. Glassmaking is next. A pinch of selenium cancels the green tinge that iron impurities give to glass, while more of it, combined with cadmium sulfide, makes the deep ruby red of traffic lights and glassware. The same cadmium sulfoselenide pigments colour plastics and ceramics.
Grey selenium's photoconductivity powered xerography: early photocopier drums were coated with it, and selenium photocells sat in camera light meters and automatic doors. Organic photoconductors and silicon have taken over most of those roles, but selenium is now in thin-film solar cells of the copper indium gallium selenide type. Selenium sulfide is the active ingredient in some anti-dandruff shampoos, and sodium selenite is added to animal feed and to multivitamins.
Reading the numbers
Selenium melts at 221 °C and boils at 685 °C, so it would melt in a very hot domestic oven and turn to vapour in a kiln. Its density of 4.81 grams per cubic centimetre is nearly five times that of water, heavier than aluminium but well short of iron. That density is for the grey form; the other allotropes are lighter.
The configuration [Ar] 3d10 4s2 4p4 gives six outer electrons, two short of a full shell, so selenium behaves like sulfur: it gains two electrons to form selenides and shares electrons to form the +4 and +6 states seen in selenite and selenate. Its electronegativity of 2.55 is a shade below sulfur's, and its first ionisation energy of 941 kJ/mol is slightly lower than arsenic's despite selenium being further right, because its fourth p electron must share an orbital and is easier to remove. Being a period 4 element under sulfur explains why the body sometimes mistakes selenium for sulfur and builds it into proteins.
Safety and the environment
Adults need roughly 55 micrograms of selenium a day, obtained easily from a mixed diet. Intakes above about 400 micrograms a day over time cause selenosis: brittle nails, hair loss, garlic breath and nerve problems. A handful of Brazil nuts can exceed the daily requirement, so they are best eaten in moderation. Hydrogen selenide gas is highly toxic, and selenium compounds in the workplace are handled with care.
Livestock grazing on selenium-rich plants develop hoof and hair disorders. In the 1980s selenium in irrigation drainage water caused deformities in birds at Kesterson Reservoir in California, a case that changed how agricultural drainage is managed in the United States. Coal ash and mine drainage are other routes into water.
Did you know?
- Selenium was discovered in the red sludge that built up in a sulfuric acid factory part-owned by Berzelius.
- Grey selenium conducts electricity far better in light than in the dark, which is why it coated the drums of the first photocopiers.
- Brazil nuts are the richest common food source of selenium, and a single nut can contain more than an adult needs in a day.
- Finland has deliberately added selenium to its fertilisers since 1984 to correct low levels in the national diet.
- One of the first signs of selenium overdose is breath that smells strongly of garlic.
- The enzyme that converts thyroid hormone into its active form contains selenium, which is why deficiency can affect metabolism.
More in the non-metals
Column-mates come first, since elements in one column have outer electrons arranged in a similar way and tend to behave alike; the rest are the nearest neighbours in the family.
See all 7 non-metals on one page, or browse all 118 elements.
Frequently asked questions
What does selenium do in the body?
Selenium is built into about twenty-five proteins called selenoproteins. These include antioxidant enzymes that protect cells from damage and the enzymes that activate thyroid hormone. It is also needed for a healthy immune system and for sperm production.
Can you take too much selenium?
Yes, and the safety margin is narrower than for most nutrients. Regular intakes above around 400 micrograms a day can cause hair loss, brittle nails, garlic breath and nerve damage. Most people get enough from food, so supplements are rarely needed.
Why is selenium in shampoo?
Selenium sulfide slows the growth of the yeast that contributes to dandruff and reduces the rate at which scalp skin cells flake off. It has been used in anti-dandruff shampoos for decades. The compound stays on the scalp and is not absorbed in meaningful amounts.
Is selenium a metal?
No, selenium is a non-metal in the same group as oxygen and sulfur. Its grey form looks metallic and conducts a little electricity, especially in light, but chemically it behaves like sulfur. It forms selenides and selenates the way sulfur forms sulfides and sulfates.
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Written by Anthony. Updated September 2026. Numerical data from Periodic-Table-JSON (CC BY-SA), cross-checked against the Selenium reference article. Spotted an error? Tell us.