Actinium - chemical symbol Ac, atomic number 89 - is a silver-white, metallic, radioactive, glow-in-the-dark chemical element that crystallizes in cubic close packing and is the first member of the element group of the so-called actinides.
The chemistry of actinium is characterized by the oxidation state +3; due to the radioactive properties and the low availability, however, only a few actinium compounds are known or have been characterized. In addition to the actinium halides, these include some oxygen compounds as well as actinium phosphate and oxalate.
Actinium only occurs in extremely small quantities in nature. For example, 1 ton of uranium ore - a major occurrence of the element - contains about 0.2 milligrams and a ton of thorium ore only 5 nanograms. Accordingly, isolating and extracting actinium from such sources is impractical and uneconomical.
Instead, the metal is produced in milligram quantities by neutron irradiation of Radium-226 in a nuclear reactor; the yield is about 2% of the weight of radium used:
The Ac atom - and thus the chemical element actinium - is clearly defined by the 89 positively charged protons in the atomic nucleus. The same number of electrons ensure electrical balance in the uncharged actinium atom.
The actinium atomic nuclei differ in the number of neutrons. These types of atoms are summarized under the term actinium isotopes or actinium nuclides (isotope data: see there).
The terrestrial actinium deposits consist of the two nuclides actinium-227 and actinium-228, of which 227 Ac is the more stable isotope. The relative atomic mass is therefore given as 227 u.
| 1s | 2s | 2p | 3s | 3p | 3d | 4s | 4p | 4d | 4f | 5s | 5p | 5d | 5f | 6s | 6p | 6d | 6f | 7s | 7p |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 2 | 2 | 6 | 2 | 6 | 10 | 2 | 6 | 10 | 14 | 2 | 6 | 10 | 2 | 6 | 1 | 2 |
Shorthand electron configuration of Actinium: [Rn] 6d1 7s2 .
The following table lists the ionization energies IE (ionization potentials); the IE is the energy required in electron volt (eV) per atom to separate a given electron from an Actinium atom.
| IE1 | IE2 |
|---|---|
| 5.17 | 11.75 |
An overview of the nuclides as well as the isotopic data and properties are listed on the following page: Actinium isotopes.
The chemistry of actinium is characterized by the three valence electrons. Due to the complete overlap of the s and d levels, these three electrons are energetically equivalent; this results - as with the other elements of the scandium group - in the preferred oxidation state +3 of actinium in the actinium compounds.
| E0 (V) | Symbol | Nox | Name Ox. Name Red. | Ox. Red. | e- |
|---|---|---|---|---|---|
| -2.20 | Ac | + III 0 | Actinium(III) cation Actinium | Ac3+ ⇄ Ac (s) | + 3 e- |
The following table lists some physical data and material properties. Unless otherwise stated, the values refer to Ac-227.
[1] - H. W. Kirby:
The Discovery of Actinium.
In: Isis, 62, 3, (1971), DOI 10.1086/350760.
[2] - Maryline G. Ferrier, Enrique R. Batista, John M. Berg et al.:
Spectroscopic and computational investigation of actinium coordination chemistry.
In: Nature Communications, 7, 12313, (2016), DOI 10.1038/ncomms12312.
Last update: 2024-08-26
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