This element was discovered by P. Cleve in Uppsala, Sweden in 1879. The name comes from Thule, an island in the North Atlantic. Thule was also the first name for Scandinavia.

Ionization energies
TmI 6.2 eV, TmII 12.1 eV, TmIII 23.7 eV, TmIV 42.7 eV.

Behavior in stars
The presence of TmI in the sun is doubtful, but TmII is present with W(3462) = 0.011.

Reynolds et al. (1988) detected TmII in one F 0Ib star, with W(3462) = 0.003. Tm is also present in one M2III star according to Davis (1947).

Behavior in non-normal stars
Tm is the least abundant element of the rare earth group. It was found in the form of Tm I in some Ap stars of the Cr-Eu-Sr subgroup by Jaschek and Brandi (1972) and Adelman (1973b). Cowley and Crosswhite (1978) and Poli et al. (1987) found TmII in two Ap stars of the Si subgroup and Aikman et al. (1979) detected Tm III in the spectrum of another Ap star. Bidelman (1953) found TmI lines in one S-type star.

Tm has one stable isotope, Tm169, and 17 short-lived isotopes and isomers.

Tm169 can be produced by both the r process and the s process.

Published in "The Behavior of Chemical Elements in Stars", Carlos Jaschek and Mercedes Jaschek, 1995, Cambridge University Press.