Predict the formula of promethium(III) oxide.
Pm3+ with O2- crosses to Pm2O3 — identical in pattern to its neighbors Nd2O3 and Sm2O3.
What Is the Charge of Promethium?
Promethium commonly forms Pm3+ in introductory ionic compounds.
Promethium follows the lanthanide rule and forms Pm3+ — pink solutions, salts like PmCl3 — but every isotope is radioactive, so its chemistry is studied in milligram amounts. It is the only lanthanide with no stable form on Earth.
| Element | Promethium |
|---|---|
| Symbol | Pm |
| Atomic number | 61 |
| Common ionic charge | +3 |
| Common ions | Pm3+ |
Visual reference
Promethium element image and atomic model
Images are used as visual references; charge rules still depend on the compound.
The electron story behind Promethium's charge
Promethium is [Xe]4f5 6s2, and the usual three-electron departure gives Pm3+ (f4). Nothing about nuclear instability alters that: electron chemistry and nuclear decay are separate ledgers, so promethium slots neatly between neodymium and samarium in every chemical trend.
This is a classroom reference range, not an exhaustive list of all oxidation states.
The assumption that trips people up on Promethium
Assuming an element 'missing from nature' must have strange chemistry. Promethium's rarity is purely nuclear — its most stable isotope lasts under 18 years, so primordial promethium decayed away eons ago. Chemically it is as ordinary a +3 lanthanide as they come.
Worked example: Pm in a formula problem
Pm3+ in real compounds
PmCl3
Promethium chloride
Pm3+ balances chloride ions.
Pm2O3
Promethium oxide
Promethium can form oxides where total positive and negative charge balances.
Questions students ask about promethium
Was promethium really used in glowing watch dials?
Yes — mid-century luminous paint mixed Pm-147 (as a +3 salt) with a phosphor, replacing the more hazardous radium. Its soft beta decay lit the phosphor without radium's penetrating gamma rays.
Where does promethium come from if it doesn't exist naturally?
Nuclear reactors: it is fished out of fission products, as Pm3+, by the same ion-exchange chemistry used to separate all the look-alike lanthanide ions.
What are nuclear batteries doing with Pm3+?
Betavoltaic cells let Pm-147's beta electrons strike a semiconductor and generate current — nuclear decay in, electricity out. The promethium sits there as an ordinary +3 compound while its nuclei do the work.
How do I use this charge in formulas?
Combine positive and negative ions so the total charge is zero — or let the ionic compound calculator do the crossing for you. The full chart lives on the periodic table with charges.
Where this data comes from
Charges and element facts on this page are checked against the primary references below. Found an error? Email hello@periodictablewithcharges.com — corrections ship in the next content update. See how this site is edited.