Wordorigins.org

Thulium, element 69, is yet another example of an element named after someplace in Scandinavia, or in this case, Scandinavia itself. The element was discovered in 1879 by Swedish chemist Per Teodor Cleve, who dubbed it thulium, after his native land; from the 12 September 1879 issue of Chemical News and Journal of Physical Science:

For the radical of the oxide placed between ytterbia and erbia [...] I propose the name of Thullium [sic], derived from Thulé, the ancient name of Scandinavia.¹