Promethium
Lanthanidesolidblock f
The only lanthanide with no stable isotope, a genuine gap in the table, recovered from fission products and once painted onto luminous dials.
| Atomic number | 61 |
| Mass number (longest-lived isotope) | 145 g/mol |
| This element has no stable isotope, so it has no standard atomic weight. The figure above is the mass number of its longest-lived isotope — a convention, not a measurement of a natural mixture. | |
| Group · period | f-block series · 6 |
| Electron configuration | [Xe] 4f⁵ 6s² |
About Promethium
Promethium is one of just two elements below bismuth with no stable isotope — technetium is the other — and the reason is a quirk of nuclear structure: an odd atomic number sandwiched between neodymium and samarium, both of which have stable isotopes at every mass number promethium would want. Its longest-lived isotope, promethium-145, has a 17.7-year half-life, so any primordial promethium vanished long ago and the few hundred grams thought to exist in the crust are transient uranium fission products. It was finally identified in 1945 at Oak Ridge in fission waste and named for Prometheus, who stole fire. Promethium-147 replaced radium in luminous paint in the 1960s as a safer beta emitter, and it still powers a handful of long-life betavoltaic cells.
Moles from Mass (n = m/M)
Opens with M = 145 g/mol already filled in
Gas Density from Molar Mass
Opens with M = 145 g/mol already filled in
Faraday's Law of Electrolysis (m = QM/nF)
Opens with M = 145 g/mol already filled in
Ion Concentration as CaCO₃ Equivalent
Opens with EW = 145 g/mol already filled in
Acid Feed to Reduce Alkalinity
Opens with EW = 145 g/mol already filled in