Atomic number 89

Actinium

Ac
+ phase
− phase

Shells Orbitals 6d 7s

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89 electrons in 7 shells: 2, 8, 18, 32, 18, 9, 2.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Actinide
Period: 7
Block: f
Atomic mass: 227.02775 Da (longest-lived isotope)
Electron configuration: [Rn] 6d¹ 7s²
Phase at 25 °C: solid
Melting point: 1227 °C
Boiling point: 3198 °C
Density: 10.07 g/cm³
Electronegativity (Pauling): 1.1
Discovered: 1899, André-Louis Debierne

Neighbours in the table

Ra Th

André-Louis Debierne, a French chemist, came upon actinium in 1899 while trying out new methods for separating rare earth oxides, in Jefferson Lab’s account. Friedrich Otto Giesel found the same element independently in 1902. The name is built on aktis, aktinos, the Greek for a beam or ray, Los Alamos records. Actinium has no stable isotope. PubChem gives the mass of its longest-lived isotope, actinium-227, as 227.028 Da.

Actinium is an f-block element, in the second of the two rows that periodic tables print at the bottom. Chem1 notes that those rows sit there only to keep the chart from growing too wide. Seven shells hold actinium’s 89 electrons, 2, 8, 18, 32, 18, 9 and 2, written [Rn] 6d¹ 7s². Its oxidation state, the charge the atom would hold if all its bonds were fully ionic, is +3 in Jefferson Lab’s listing, with +2 as well in Los Alamos’s. In chemical behaviour actinium resembles the rare earths, lanthanum most of all, Los Alamos observes.

Ionisation energy is the energy needed to remove an electron from an atom. For actinium’s first electron it is 5.17 eV by Jefferson Lab’s figure and 5.380235 eV in NIST’s measurement. The metal melts at 1051 °C on Jefferson Lab’s figure and at 1227 °C on Los Alamos’s.

Where it is found

Actinium occurs with uranium minerals, Los Alamos records, and actinium-227 is itself a decay product of uranium-235. The quantities in ore are tiny. Actinium makes up about 5.5 × 10⁻¹⁰ milligrams per kilogram of the Earth’s crust, by Jefferson Lab’s estimate. With so little in the ground, making actinium on demand is usually cheaper and easier: radium is bombarded with neutrons in a nuclear reactor, in Jefferson Lab’s account.

Metallic actinium has been obtained by reducing actinium fluoride with lithium vapour at roughly 1100 to 1300 °C, Los Alamos reports. A freshly purified sample does not stay pure for long. After 185 days it reaches equilibrium with its decay products, according to Los Alamos. From then on it decays at the pace of its own half-life, the time in which half of a sample decays. Thorium-227, with an 18.5-day half-life, and radium-223, with an 11.4-day half-life, are its chief decay products. Short-lived isotopes of radon, bismuth, polonium and lead follow them down the chain.

Why it matters

Actinium’s activity is roughly 150 times radium’s, by Los Alamos’s estimate, which makes it useful for producing neutrons. The half-life of actinium-227 is 21.772 years in the evaluation of the IAEA, the International Atomic Energy Agency. By IAEA’s figures, 98.62 per cent of its decays are beta-minus emissions and 1.38 per cent alpha emissions. The beta branch leaves thorium-227 and the alpha branch francium-223, in Jefferson Lab’s account. In equilibrium with its decay products, actinium is a powerful source of alpha particles, Los Alamos notes.

Actinium-227 has served as a tracer of deep-sea mixing, IUPAC’s notes explain. Measuring actinium-227 at different depths in a water column shows the rates and patterns of vertical exchange. It has also traced fluids moving in mid-ocean ridges and basaltic melts.

Actinium-225, a pure alpha emitter, is used in cancer treatment. Its half-life is 9.9190 days in the table of the IAEA Atomic Mass Data Center. A chelating agent, a substance that forms several bonds to a single metal ion, holds the isotope and delivers it to the tumour. Alpha particles from actinium and its decay products then kill the tumour cells. In April 2012 Los Alamos announced a project to produce actinium-225 rapidly in large amounts.

Properties

Properties of actinium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	89
Atomic mass	227.02775 Da	Da	PubChem, longest-lived isotope
Melting point	1227 °C	°C	Los Alamos: 1227 °C
Boiling point	3198 °C	°C	Los Alamos: 3198 °C
Density	10.07 g/cm³	g/cm³
Electronegativity (Pauling)	1.1
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 6d¹
Phase at 25 °C	solid

This element has no standard atomic weight; the atomic mass shown is that of its longest-lived isotope, as PubChem gives it.

Safety

Actinium has no stable isotope, so every sample is radioactive, and it gives off alpha, beta and gamma radiation as it decays, according to Haz-Map’s radionuclide summary in PubChem. Haz-Map rates the internal toxicity, the harm done once actinium is inside the body, as very high for actinium-227 and actinium-225 and high for actinium-228. The PubChem Laboratory Chemical Safety Summary for actinium is the reference sheet for anyone handling it.

Questions

Is actinium radioactive?

Yes. Actinium has no stable isotope, and its known isotopes run from actinium-205 to actinium-237 in PubChem's list. Actinium-227 lasts longest, with a half-life of 21.772 years in IAEA's evaluation. Actinium-228 decays wholly by beta-minus emission; its half-life is 6 hours according to Haz-Map and 6.15 hours in the table of the IAEA Atomic Mass Data Center (AMDC). It also gives off gamma rays, according to Haz-Map.

What is actinium used for?

Actinium has no significant commercial use apart from producing neutrons, according to Jefferson Lab, a job its strong radioactivity suits: about 150 times that of radium, by Los Alamos's estimate. Researchers have followed deep-ocean mixing with actinium-227 and studied fluids in mid-ocean ridges with it. Actinium-225 goes into cancer treatment and also serves as the parent in generators that supply bismuth-213.

How does actinium-225 kill cancer cells?

Bound to a chelating agent, actinium-225 is carried to the tumour, where the alpha particles released by actinium and its daughter isotopes cause the cancer cells to die, according to IUPAC's notes. One of those daughters, bismuth-213, mostly decays by beta emission to polonium-213, a very short-lived alpha emitter. The alpha particle from polonium-213 is what makes bismuth-213 toxic to cells.

What does the name actinium mean?

The name goes back to the Greek aktis, aktinos, meaning a beam or ray, Los Alamos records. André-Louis Debierne reported actinium in 1899 while testing new ways to part rare earth oxides from one another, and Friedrich Otto Giesel arrived at the same element on his own three years later.