Promethium (Pm)
Lanthanide · f-block · period 6
Silvery metal that glows faintly blue-green from its own radioactivity.
Promethium (symbol Pm) is chemical element number 61, a lanthanide in the lanthanide series (f-block), period 6 of the periodic table. Its standard atomic weight is (145) and its electron configuration is [Xe] 4f5 6s2. At room temperature it is a solid, and it is radioactive. The only radioactive lanthanide, and the rarest — barely a kilogram exists in the entire Earth's crust.
Properties
| Atomic number | 61 |
|---|---|
| Atomic mass | (145) |
| Category | Lanthanide |
| Block | f-block |
| Group · Period | f-block series · 6 |
| Electron configuration | [Xe] 4f5 6s2 |
| Electrons per shell | 2, 8, 18, 23, 8, 2 |
| State at 25 °C | solid |
| Melting point | 1315 K |
| Boiling point | 3273 K |
| Density | 7.26 g/cm³ |
| Electronegativity (Pauling) | 1.13 |
| Radioactive | Yes — no stable isotope |
| Origin | decay |
| Discovered | 1945 · Marinsky, Glendenin & Coryell |
Key facts about Promethium
- The only radioactive lanthanide, and the rarest — barely a kilogram exists in the entire Earth's crust.
- Named after Prometheus, who stole fire from the gods.
- It fills the gap at atomic number 61 that puzzled early periodic-table makers.
- Once used in glow-in-the-dark watch dials.
What Promethium is used for
Nuclear-powered batteries for pacemakers and spacecraft, luminous paint (historic), thickness gauges.
Where it sits in the periodic table
Promethium is in the lanthanide series, period 6, of the f-block. Its electron configuration [Xe] 4f5 6s2 means its outermost shell holds 2 electrons, which is what places it here and explains much of its chemistry. See Promethium on the interactive table →
Promethium — frequently asked questions
What is Promethium used for?
Nuclear-powered batteries for pacemakers and spacecraft, luminous paint (historic), thickness gauges.
Is Promethium radioactive?
Yes. Promethium has no stable isotope, so every form of it is radioactive.
What is the electron configuration of Promethium?
Promethium has the electron configuration [Xe] 4f5 6s2, which is 2, 8, 18, 23, 8, 2 electrons per shell.
What group and period is Promethium in?
Promethium is in the lanthanide series (f-block), period 6, in the f-block. Its outer shell holds 2 electrons.
Who discovered Promethium?
Promethium was discovered in 1945 by Marinsky, Glendenin & Coryell.