Actinium
chemical element with symbol Ac and atomic number 89
Actinium is a chemical element; it has symbol Ac and atomic number 89. It was discovered by Friedrich Oskar Giesel in 1902, who gave it the name emanium; the element got its name by being wrongly identified with a substance André-Louis Debierne found in 1899 and called actinium. The actinide series, a set of 15 elements between actinium and lawrencium in the periodic table, is named after actinium. Together with polonium, radium, and radon, actinium was one of the first non-primordial radioactive elements to be discovered.
A soft, silvery-white radioactive metal, actinium reacts rapidly with oxygen and moisture in air, forming a white coating of actinium oxide that prevents further oxidation. As with most lanthanides and many actinides, actinium assumes oxidation state +3 in nearly all its chemical compounds. Actinium is found only in traces in uranium and thorium ores as the isotope 227Ac, which decays with a half-life of 21.772 years, predominantly emitting beta and sometimes alpha particles, and 228Ac, which is beta active with a half-life of 6.15 hours. One tonne of natural uranium in ore contains about 0.2 milligrams of actinium-227, and one tonne of thorium contains about 5 nanograms of actinium-228. The close similarity of physical and chemical properties of actinium and lanthanum makes the separation of actinium from the ore impractical. Instead, the element is prepared, in milligram amounts, by the neutron irradiation of 226Ra in a nuclear reactor.
“Actinium” enters the record as chemical element with symbol Ac and atomic number 89. Crown Archives preserves that source wording while asking what Actinium, chemical and element can confirm, complicate or overturn.
Why this record matters
“Actinium” is worth following because a concise public description often conceals a longer documentary argument. Here, Actinium, chemical and element provides the most credible route into that argument.
The date and method of observation matter as much as the stated conclusion, especially where classification or consensus has changed. The source revision retrieved here is dated Aug 12, 2026. The linked authority identifier is Q1121. The Library of Congress control number is sh85000706. 1 of 1 selected statements include explicit references; 0 carry qualifiers and 0 use preferred rank. The first chronological checks are 1902 and 1899.
Scientific names, classifications and consensus can change while older terminology persists in catalogues and historical literature. The lead is largely declarative, so disagreement and counter-evidence require a deliberate search beyond the opening account. Authority statements aid reconciliation but still require their own references, qualifiers and ranks to be checked.
How to read it
Check terminology, classification and the date of the cited evidence. Scientific names and technical consensus can change while older records retain historical value.
- Current terminology
- Classification context
- Finding cited technical literature
Primary datasets, specimen catalogues, standards bodies and the most recent peer-reviewed literature.
Three-step research path
- Establish the record: confirm the title “Actinium”, its source revision and the description used here.
- Expand the search: follow Actinium primary sources, Actinium archive and Actinium research across catalogues and specialist indexes.
- Test the account: compare the strongest cited source with the responsible institution’s current record and note any disagreement.
Questions for further research
- Which source most directly establishes the central claim about “Actinium”?
- Is the terminology current, historical or disputed?
- Has classification or technical consensus changed since the cited source?
Search terms from this dossier
This entry incorporates text from “Actinium” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.