Promethium (symbol Pm, atomic number 61) is the only radioactive lanthanide, once used in luminous paint. It is classified as a lanthanide and sits in the lanthanide series (f-block), period 6 of the periodic table.
Overview
Promethium is the chemical element with atomic number 61 and the symbol Pm. With a standard atomic weight of [145], 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. Promethium is the only radioactive lanthanide, once used in luminous paint.
Atomic structure
An atom of Promethium has 61 protons in its nucleus and, when neutral, 61 electrons arranged in the configuration [Xe] 4f5 6s2. Promethium has no stable isotope; the value [145] refers to the mass number of its most stable or most commonly produced isotope.
Discovery and place in the table
Promethium was discovered in 1945 by Charles Coryell, Jacob Marinsky, Lawrence Glendenin. It follows Neodymium (Nd) and precedes Samarium (Sm) in order of atomic number.
| Atomic number | 61 |
|---|---|
| Symbol | Pm |
| Atomic mass | [145] |
| Category | Lanthanide |
| Group | — |
| Period | 6 |
| Electron configuration | [Xe] 4f5 6s2 |
| Phase at STP | Solid |
| Discovered | 1945 (Charles Coryell, Jacob Marinsky, Lawrence Glendenin) |