Europium (Eu)
“The rare earth named after an entire continent — and the glowing phosphor that guards euro banknotes from counterfeiters.”
How its electrons are arranged
Computed from the aufbau principle, with known real-world exceptions applied automatically.
The notes
Characteristics
- The most reactive lanthanide: it oxidises in air within minutes and fizzes in water, which is why it is always stored under oil.
- Unusually for a rare earth, europium lives happily in two oxidation states — Eu³⁺ and Eu²⁺ — and both ions are superb phosphors.
- Electron configuration [Xe] 4f⁷ 6s²: losing the 6s pair plus one 4f electron gives Eu³⁺, while losing only the 6s pair leaves Eu²⁺ with a tidy half-filled 4f⁷ shell.
- Its ions soak up ultraviolet light and re-emit it as brilliant red (Eu³⁺) or blue (Eu²⁺) — the trick behind its roles in lamps and security printing.
A soft, silvery-white metal — the most reactive of all the lanthanides. It tarnishes in air within minutes, so laboratories keep it under mineral oil or in sealed ampoules, where a freshly cut surface gleams like bright pewter.
Where you meet it
✦ Uses
- The red phosphor Y₂O₃:Eu³⁺ inside fluorescent tube lights and CFL bulbs — europium paints the warm red that makes lamp light feel natural.
- Blue-emitting Eu²⁺ phosphors completing the red–green–blue trio of trichromatic lamps and some modern LED lights.
- Security printing: euro banknotes carry europium-based inks and fibres that glow red and blue under a bank-teller’s UV lamp.
- Colour phosphors of old CRT television screens, where europium helped build the picture tube’s glowing colours.
◍ Everyday examples
- Switch on a classroom tube light or CFL bulb — europium’s red phosphor is glowing right now inside the white coating.
- Any euro banknote under a UV torch at a bank or money-changer flashes europium’s hidden red and blue glow.
History & name
In 1896 the French chemist Eugène-Anatole Demarçay, checking the purity of a samarium sample, found spectral lines that belonged to no known element. It took him five more years of painstaking fractional crystallisation before, in 1901, he isolated the culprit and named it europium after his home continent, Europe. It was one of the last lanthanides to be tamed, closing a European century of rare-earth detective work.