Thulium (Tm)
Element 69 · Lanthanide · Solid at 25 °C · Period 6 (f-block)
Thulium is the rarest stable rare-earth element; one isotope serves as the X-ray source in portable radiography units.
On the periodic table, thulium sits in the f-block in period 6. It is one of 15 elements classified as a lanthanide, and at room temperature (25 °C) it is a solid.
The lanthanides — often called rare-earth elements — fill the 4f orbitals along the detached row at the bottom of the table. Chemically they are remarkably alike, almost all preferring the +3 oxidation state, which historically made them very hard to separate. Today they are anything but obscure: strong permanent magnets, screen phosphors, and lasers depend on them.
Properties of thulium
| Atomic number (Z) | 69 |
|---|---|
| Symbol | Tm |
| Atomic mass | 168.93 |
| Category | Lanthanide |
| Group | — (f-block, associated with group 3) |
| Period | 6 |
| Block | f-block |
| Metal / nonmetal | Metal |
| State at 25 °C | Solid |
| Electron configuration | [Xe] 4f13 6s2 |
| Electronegativity (Pauling) | 1.25 |
| Melting point | 1,545 °C (1,818.15 K / 2,813 °F) |
| Boiling point | 1,950 °C (2,223.15 K / 3,542 °F) |
| Density | 9.32 g/cm³ |
Position in the periodic table
Thulium belongs to the lanthanides, the detached f-block row shown beneath the main table. These elements all fit between groups 2 and 4 of period 6, and are pulled out into their own row only to keep the table a manageable width.
Physical properties
Thulium melts at 1,545 °C (1,818.15 K / 2,813 °F) and boils at 1,950 °C (2,223.15 K / 3,542 °F), giving it a liquid range of about 405 °C at standard pressure.
Its density is 9.32 g/cm³.
See also: the interactive periodic table, how to read the periodic table, periodic trends.