Actinium (Ac)

Actinium (Ac) has atomic number 89 and is the first element of the actinide series. It appears below lanthanum and marks the start of the f-block elements commonly called the actinides.

All known isotopes of actinium are unstable. The nuclei are large and energetically favor decay to reach more stable configurations. Common decay modes include alpha and beta decay, for example:

• Beta decay: \(^{227}\mathrm{Ac} \to ^{227}\mathrm{Th} + \beta^- + \bar{\nu}_e\)
• Alpha decay: \(^{225}\mathrm{Ac} \to ^{221}\mathrm{Fr} + \alpha\)

The two most discussed isotopes are:

• \(^{227}\mathrm{Ac}\): longest-lived, half-life ≈ 21.8 years; decays primarily by beta emission.
• \(^{225}\mathrm{Ac}\): half-life ≈ 10 days; an alpha emitter used in radiopharmaceutical research (targeted alpha therapy).

Ground-state electron configuration is often written as [Rn] 6d1 7s2. In compounds, actinium almost exclusively shows the +3 oxidation state (\(\mathrm{Ac}^{3+}\)), which has the noble-gas-like core [Rn].

Natural actinium occurs in trace amounts in uranium ores. For research/medical use, \(^{225}\mathrm{Ac}\) is typically produced via accelerator routes (e.g., spallation of \(^{232}\mathrm{Th}\) with high-energy protons) or harvested from the decay of \(^{229}\mathrm{Th}\).

Historically, actinium served as a laboratory radiation source. Today, \(^{225}\mathrm{Ac}\) is being actively studied for targeted alpha therapy (TAT) in oncology, where its short-range, high-linear-energy-transfer \(\alpha\)-particles can selectively damage tumor cells.

Yes. As a strong emitter (especially alpha for \(^{225}\mathrm{Ac}\)), it poses a significant internal hazard if inhaled or ingested. Handling requires licensed facilities, glove boxes, fume hoods, remote tools, and strict radiological monitoring and waste controls.

In the +3 state, actinium forms salts analogous to lanthanides, such as actinium(III) chloride (\(\mathrm{AcCl_3}\)), actinium(III) nitrate (\(\mathrm{Ac(NO_3)_3}\)), and actinium(III) oxide (\(\mathrm{Ac_2O_3}\)).

Actinium (start of actinides) parallels lanthanum (start of lanthanides) in showing a dominant +3 state and forming similar ionic compounds. However, actinium’s isotopes are all radioactive, and 5f/6d orbital participation becomes increasingly important deeper into the actinide series.

A representative example is the \(^{227}\mathrm{Ac}\) chain: \(^{227}\mathrm{Ac} \xrightarrow{\beta^-} ^{227}\mathrm{Th} \xrightarrow{\alpha} ^{223}\mathrm{Ra} \;\cdots\). Such chains continue until a stable nuclide (often lead) is reached.