Thulium (symbol Tm, atomic number 69) is the rarest stable rare-earth metal, used in portable X-ray sources. It is classified as a lanthanide and sits in the lanthanide series (f-block), period 6 of the periodic table.
Overview
Thulium is the chemical element with atomic number 69 and the symbol Tm. With a standard atomic weight of 168.93, it is a lanthanide. In the periodic table it occupies the lanthanide series (f-block) and period 6, and under standard conditions it is normally a solid. Thulium is the rarest stable rare-earth metal, used in portable X-ray sources.
Atomic structure
An atom of Thulium has 69 protons in its nucleus and, when neutral, 69 electrons arranged in the configuration [Xe] 4f13 6s2. Its standard atomic weight of 168.93 reflects the natural mix of its stable isotopes.
Discovery and place in the table
Thulium was discovered in 1879 by Per Teodor Cleve. It follows Erbium (Er) and precedes Ytterbium (Yb) in order of atomic number.
| Atomic number | 69 |
|---|---|
| Symbol | Tm |
| Atomic mass | 168.93 |
| Category | Lanthanide |
| Group | — |
| Period | 6 |
| Electron configuration | [Xe] 4f13 6s2 |
| Phase at STP | Solid |
| Discovered | 1879 (Per Teodor Cleve) |