Terbium (Tb)
“The rare earth that flashes brilliant green under ultraviolet light — and stretches like tiny muscle in a magnetic field.”
How its electrons are arranged
Computed from the aufbau principle, with known real-world exceptions applied automatically.
Characteristics
- Electron configuration [Xe] 4f⁹ 6s² — nine electrons buried in the 4f sub-shell; shedding the 6s pair and one 4f electron leaves the Tb³⁺ ion that does nearly all terbium chemistry.
- Tb³⁺ soaks up ultraviolet energy and releases it as a brilliant green glow near 545 nanometres — the sharpest, most celebrated green among the rare earths.
- One of very few lanthanides that can also wear a +4 oxidation state — dark-brown terbium(IV) oxide (TbO₂) is the textbook example, since most terbium compounds stop at +3.
- Cooled well below room temperature, terbium atoms line up into a ferromagnet, and its Tb³⁺ ions carry one of the largest magnetic moments of any ion — about 9.5 Bohr magnetons.
A soft, silvery-white metal, soft enough to slice with a knife, which tarnishes only slowly in air like its lanthanide cousins. Its compounds are the giveaway: under invisible ultraviolet light, even traces of Tb³⁺ flash a brilliant, unmistakable green.
Where you meet it
✦ Uses
- The green phosphor of trichromatic fluorescent lamps and CRT screens — terbium-activated cerium magnesium aluminate paints the green that makes lamp light feel natural.
- Terfenol-D, an alloy of terbium, dysprosium and iron, stretches and shrinks in magnetic fields — the “muscle” of naval sonar transducers and of speakers that turn a tabletop into a soundboard.
- Green X-ray scintillator screens of terbium-activated gadolinium oxysulfide (Gd₂O₂S:Tb), converting invisible rays into light a radiographer can read.
- Fluorescent tag for security inks and speciality glasses, because terbium’s unmistakable green glow is almost impossible to counterfeit.
◍ Everyday examples
- The pleasant, natural feel of a trichromatic tube light’s white glow owes a hidden slice of its spectrum to a terbium phosphor working inside the coating.
- Naval ships and survey vessels probing the seabed project their sound through terbium-rich Terfenol-D drivers — sonar “voices” made of metal muscle.
History & name
Terbium was discovered in 1843 by the Swedish chemist Carl Gustaf Mosander, who found it hiding as an impurity in yttrium oxide prepared from gadolinite — a heavy black mineral quarried at Ytterby, near Stockholm. Splitting that oxide, he announced two new elements, terbium and erbium, but pinned their names on the wrong coloured fractions; it took chemists the following decades to untangle the muddle and swap the names into today’s meanings. Genuinely pure terbium only arrived in the twentieth century, when ion-exchange chemistry finally separated the look-alike lanthanides.