Selenium (Se)
“The Moon-named element whose love of light powers anti-dandruff shampoo, the first photocopiers and the ancestor of every solar panel.”
How its electrons are arranged
Computed from the aufbau principle, with known real-world exceptions applied automatically.
Characteristics
- A nonmetal of the oxygen family — group 16, period 4, p-block, configuration [Ar] 3d¹⁰ 4s² 4p⁴, six valence electrons — sulfur’s downstairs neighbour and tellurium’s upstairs one.
- Light-hungry: grey selenium drops its electrical resistance sharply under illumination, the photoconductivity that ran light meters, photocopier drums and the first solar cells.
- Nutrient and poison in the same bottle: the body needs roughly 55 micrograms a day inside antioxidant enzymes, yet a steady surplus brings selenosis — garlic breath, brittle nails, hair falling out.
- Burning in air, it gives selenium dioxide (SeO₂) with a horseradish-like stench — the very smell that greeted Berzelius — and in compounds it mirrors sulfur, forming metal selenides such as the thin-film solar material CIGS.
Selenium wears several faces: a red amorphous powder, a black glassy form, and — most important — grey, metallic-looking crystals. The grey allotrope resembles a dull metal yet behaves as a semiconductor, and it is this form that devices use; the red powder is what chemists meet first when elemental selenium precipitates.
Where you meet it
✦ Uses
- Selenium sulfide (SeS₂) shampoo — the anti-dandruff classic, Selsun and its kin — slows the Malassezia yeast whose feasting flakes the scalp.
- The first photocopiers: Xerox’s original machines ran on drums coated with amorphous selenium, whose charged, light-struck surface herded toner into crisp copies.
- The glass-maker’s secret: a pinch of selenium erases the green tint that iron leaves in glass; a bigger dose stains glass rose-red, and cadmium selenide makes fire-engine-red pigments.
- Grandparent of solar power: Charles Fritts’s 1883 selenium cell was the first solid plate to turn sunlight into electric current — ancestor of every rooftop panel today.
- Feed and supplements: selenium yeast and sodium selenite top up poultry and livestock feed, guarding animals against selenium-deficiency diseases.
◍ Everyday examples
- Check the medicine shelf: the anti-dandruff shampoo bottle lists “selenium sulfide” right on its label.
- Brazil nuts are the selenium champions of the food world — a single nut can carry a whole day’s allowance, while rice from poor soils carries far less.
- Vintage camera light meters with a honeycomb window needed no battery — a selenium strip made electricity straight from light.
History & name
In 1817 the great Swedish chemist Jöns Jacob Berzelius noticed a brick-red crust at the bottom of a sulphuric-acid plant near Gripsholm; other chemists had shrugged it off, but he analysed it and found, besides sulfur, a new substance whose oily residue burned with a horseradish stench. He at first mistook it for the recently discovered tellurium — the “Earth element” — but the numbers refused to match, so he announced a new element and named it selenium, from the Greek selēnē, the Moon, as tellurium’s celestial twin. Berzelius nearly poisoned himself breathing the fumes, and for decades selenium sat as a laboratory curiosity — until 1873, when Willoughby Smith discovered that its conductivity leaps when light falls on it, and in 1883 Charles Fritts built history’s first solar cell on a selenium wafer.