thulium

American  

[thoo-lee-uhm] / ˈθu li əm /

noun

Chemistry.

1. a rare-earth metallic element found in the minerals euxenite, gadolinite, etc. Tm; 168.934; 69; 9.32.

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thulium British  

/ ˈθjuːlɪəm /

noun

1. a malleable ductile silvery-grey element occurring principally in monazite. The radioisotope thulium-170 is used as an electron source in portable X-ray units. Symbol: Tm; atomic no: 69; atomic wt: 168.93421; valency: 3; relative density: 9.321; melting pt: 1545°C; boiling pt: 1950°C

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thulium Scientific  

/ tho̅o̅′lē-əm /

1. A soft, silver-gray metallic element of the lanthanide series. One of its artificial radioactive isotopes is used as a radiation source in small, portable x-ray machines. Atomic number 69; atomic weight 168.934; melting point 1,545°C; boiling point 1,727°C; specific gravity 9.3; valence 2, 3.

2. See Periodic Table

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Etymology

Origin of thulium

From New Latin, dating back to 1875–80; Thule, -ium

Example Sentences

Heavy rare-earth elements include dysprosium, terbium, holmium, erbium, thulium, ytterbium, lutetium, and yttrium.

From Barron's • Nov. 13, 2025

The 17 elements are: lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, scandium, yttrium.

From Reuters • Jul. 5, 2023

Ultra Safe has explored different isotopes, like cobalt-60 and thulium, which can be scaled to produce 10 times the energy of traditional plutonium systems while beingsafer and more cost-effective.

From Seattle Times • Jun. 26, 2022

The 794 nm photon interacts with a lithium-niobate crystal doped with thulium, so that the photon’s state becomes stored in the crystal.

From Scientific American • Jun. 19, 2019

Who, then, will be the first to discover a use for indium, germanium, terbium, thulium, lanthanum, neodymium, scandium, samarium and others as unknown to us as tungsten was to our fathers?

From Creative Chemistry Descriptive of Recent Achievements in the Chemical Industries by Slosson, Edwin E.