Terbium

Terbium is a soft, silvery lanthanide with the configuration [Xe]4f9 6s2. It normally forms Tb3+, but unlike most lanthanides it can also reach the +4 state, because removing one more electron leaves the stable half-filled 4f7 shell. That is why heating terbium in air gives the dark brown mixed oxide Tb4O7, which contains both Tb3+ and Tb4+, rather than a simple Tb2O3.

The Tb3+ ion emits a strong, pure green light near 545 nanometres when excited by ultraviolet radiation or electrons. Terbium-activated phosphors supply the green component in tri-phosphor fluorescent lamps, and since the eye is most sensitive to green, this one element provides much of the lamp's apparent brightness. Combined with dysprosium and iron in the alloy Terfenol-D, terbium also gives the largest room-temperature magnetostriction known: the material visibly changes length in a magnetic field.

Discovery

In 1843 Swedish chemist Carl Gustaf Mosander split the earth yttria into three fractions, which he called yttria, erbia and terbia, all named after the village of Ytterby near Stockholm, whose quarry supplied the ore. Later chemists swapped the names of erbia and terbia, so the substance we now call terbium was originally Mosander's 'erbia'. Terbium was not obtained in reasonably pure form until ion-exchange methods appeared in the 1950s.