It was named after the Moon. In 1817, Swedish chemist Berzelius discovered a new element in sulfuric acid residue and named it Selene — after the Greek goddess of the Moon. A poetic choice: the neighbor of tellurium (named after Earth) received the name of our closest celestial companion.
Selenium is one of the most paradoxical elements. Essential for life, yet deadly in excess. A semiconductor that "wakes up" in light. Without it, your thyroid and immune system cannot function. And it gives glass a stunning ruby-red color.
In nature, selenium occurs mainly as a trace impurity in copper and nickel sulfide ores. It exists in several allotropic forms: gray (crystalline, semiconducting), red (amorphous), and black (vitreous). Gray selenium is the most stable form and the one that exhibits photoconductivity.
Selenium is a component of 25 selenoproteins that protect cells from oxidative stress. Brazil nuts are the richest natural source of this essential micronutrient.
Selenium is essential for health but toxic in excess. Chronic intake above 400 μg/day causes selenosis: hair loss, brittle nails, and a characteristic garlic-like breath odor. Hydrogen selenide (H₂Se) is an extremely poisonous gas with a pungent smell. Respirators and exhaust ventilation are mandatory when working with selenium dust.
Selenium's electrical conductivity increases 1,000-fold when exposed to light. Willoughby Smith discovered this in 1873 — and it paved the way for solar cells.
Selenium is named after the Moon (Greek Selene). Berzelius chose this name because the element was similar to tellurium, named after the Earth (Latin Tellus).
Selenium has one of the narrowest safety margins of any element: beneficial at 55 μg/day, toxic above 400 μg/day. That's only a 7-fold difference.
A single Brazil nut contains up to 95 μg of selenium — nearly twice the daily requirement. Just two nuts a day fully cover your body's needs.
The first photocopiers worked thanks to selenium. An amorphous selenium drum formed images under light — that's how xerography was born in 1938.
| Isotope | Mass (u) | Abundance | Half-life | Decay |
|---|---|---|---|---|
74Se | 73.922476 | 0.89% | stable | — |
76Se | 75.919214 | 9.37% | stable | — |
77Se | 76.919914 | 7.63% | stable | — |
78Se | 77.917309 | 23.77% | stable | — |
80Se | 79.916521 | 49.61% | stable | — |
82Se | 81.916699 | 8.73% | stable | — |
Analysis of sulfuric acid residue