Illustration of Thulium

1879 Thulium

The book of science

Tom Sharp

UppsalaPer Teodor Cleve elements Illustration of Thulium


Thulium’s chemical symbol, Tm, is not a typo for Tin. Per Teodor Cleve discovered it looking for impurities in rare earth oxides. He found two impurities in erbia that he called holmia and thulia. Holmia was the brown oxide and thulia was the green one. He named holmia after Holmia (Stockholm) and thulia after Thule (Scandinavia).

Atomic number 69

Holmium, chromium, and thulium make an efficent laser. Radioactive thulium isotopes power portable X-ray devices. Thulium fluoresces blue to help identify euro banknotes.


It’s not the rarest rock on earth. It’s not the weirdest bird to swim or fly. I’m not the hairiest biped in the woods. I’m not the smartest primate but I try.

Thulium is the second rarest element, after promethium, in the lanthanide group of the periodic table. Sitting at the table, one would never say, “Please pass the thulium.” By which I mean to say that most of us go through life without using “thulium” in a sentence.

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