Predict the formula of actinium(III) oxide.
Ac3+ with O2- crosses to Ac2O3 — the lanthanum pattern exactly, verified by radiochemists on samples too small to see.
What Is the Charge of Actinium?
Actinium commonly forms Ac3+ in introductory ionic compounds.
Actinium is Ac3+ — exclusively. Like lanthanum, whose series-naming role it mirrors, it has one charge, colorless solutions, and textbook M2O3/MCl3 formulas. Fiercely radioactive, it is studied in milligrams and glows blue in the dark from its own decay.
| Element | Actinium |
|---|---|
| Symbol | Ac |
| Atomic number | 89 |
| Common ionic charge | +3 |
| Common ions | Ac3+ |
Visual reference
Actinium element image and atomic model
Images are used as visual references; charge rules still depend on the compound.
The electron story behind Actinium's charge
Actinium is [Rn]6d1 7s2 — lanthanum's configuration one row down. All three outer electrons depart for the radon core, and with no f electrons in play, no alternative state ever competes. The elements after it bury electrons into 5f orbitals, which is why the series carries its name.
This is a classroom reference range, not an exhaustive list of all oxidation states.
The assumption that trips people up on Actinium
Expecting actinide charge chaos to start at actinium. The famous multi-state complexity (uranium's four states, plutonium's five) begins further in; actinium itself is as single-minded as any element in the table — a fixed 3+ bookend before the storm.
Worked example: Ac in a formula problem
Ac3+ in real compounds
AcCl3
Actinium chloride
Ac3+ balances chloride ions.
Ac2O3
Actinium oxide
Actinium can form oxides where total positive and negative charge balances.
Questions students ask about actinium
Why is Ac-225 called a 'targeted alpha therapy' star?
It fires four alpha particles down its decay chain — a cluster bomb for tagged cancer cells — while its Ac3+ chemistry lets it ride the same chelator-antibody vehicles developed for lutetium. Global supply is measured in millicuries, hence the scramble to make more.
Does actinium really glow?
Faint blue in darkness — its radiation ionizing surrounding air — one of the few elements that visually announces its decay. The glow is nuclear; the 3+ chemistry is silent.
How was actinium found before anyone could see it?
Debierne (1899) tracked it through pitchblende residues by its radioactivity alone, chemistry's first decade of 'weighing the invisible' — the same tracer methods still used for its element today.
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.