| Symbol | Tm |
| Atomic number | 69 |
| Group | - |
| Period | 6 |
| Block | f |
| Classification | Lanthanide |
| Appearance | Silvery gray |
| Color | Silver |
| Number of protons | 69 p+ |
| Number of neutrons | 100 n0 |
| Number of electrons | 69 e- |
| Phase at STP | Solid |
| Density | 9.32 g/cm3 |
| Atomic weight | 168.934 u |
| Melting point | 1818 K 1544.85 °C 2812.73 °F |
| Boiling point | 2223 K 1949.85 °C 3541.73 °F |
| Heat of vaporization | 191 kJ/mol |
| Electronegativity (Pauling Scale) | 1.25 |
| Electron affinity | 99 kJ/mol |
| Oxidation states | 0, +2, +3 (a basic oxide) |
| Ionization energies |
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| Discovery and first isolation | Per Teodor Cleve (1879) |
Discovery of thulium Thulium was discovered by Swedish chemist Per Teodor Cleve in 1879 by looking for impurities in the oxides of other rare earth elements (this was the same method Carl Gustaf Mosander earlier used to discover some other rare earth elements). Cleve started by removing all of the known contaminants of erbia (Er2O3). Upon additional processing, he obtained two new substances; one brown and one green. The brown substance was the oxide of the element holmium and was named holmia by Cleve, and the green substance was the oxide of an unknown element. Cleve named the oxide thulia and its element thulium after Thule, an Ancient Greek place name associated with Scandinavia or Iceland. Thulium's atomic symbol was initially Tu, but later changed to Tm. | |