Actinium (Ac)
Actinium (symbol Ac) is the chemical element with atomic number 89: an actinide, solid at 0 °C, radioactive, with an atomic mass of 227 u. Discovered in 1899 (André-Louis Debierne).
Actinium gave its name to the whole actinide family, the bottom row of the periodic table. It is a highly radioactive metal, so rare that it is only obtained in milligrams. Long a mere lab curiosity, it has become promising in medicine: actinium-225 is used to destroy cancer cells with pinpoint precision.
Key facts
- Atomic number
- 89
- Symbol
- Ac
- French name
- Actinium
- Atomic mass
- 227 u
- Molar mass
- [227] g/mol
- Family
- Actinides
- Position
- period 7, group 3, f-block
- State at 0 °C
- Solid
- Melting point
- 1,226.9 °C
- Boiling point
- 3,226.9 °C
- Density
- 10.07 g/cm³
- Electronegativity (Pauling)
- 1.1
- First ionization energy
- 499 kJ/mol
- Atomic radius
- 195 pm
- Electron configuration
- [Rn] 6d¹ 7s²
- Abundance in Earth's crust
- 0 ppm (86th most abundant element)
- Indicative price
- 29,000 billion $/kg (order of magnitude)
- Radioactive
- yes
- Discovery
- 1899, André-Louis Debierne
Origin of the name
From the Greek aktis, ray.
What does actinium look like?
A soft, silvery metal. Its radioactivity is so intense that it excites the surrounding air, making it glow a pale blue in the dark.
Where is actinium found?
Found in trace amounts in uranium ores, where it is born from the decay of uranium-235: about 0.2 milligrams per tonne of uranium. Its most stable isotope, actinium-227, has a half-life of 21.8 years.
Natural sources
- Traces in uraninite (pitchblende)
How is actinium produced?
It is mostly made by irradiating radium-226 with neutrons in a nuclear reactor. Medical actinium-225 is harvested from the decay of thorium-229 (from old stocks of uranium-233) or produced with particle accelerators.
Top producers
- United States
- Russia
- Germany (European research center in Karlsruhe)
What is actinium used for?
- Targeted alpha therapy against certain cancers (actinium-225)
- Neutron source when mixed with beryllium
- Tracer for studying deep ocean currents
- Research into actinide chemistry
In everyday life
- No everyday objects
- Cancer clinical trials (radioactive drugs)
Role in living things
No biological role. If it enters the body, it settles mainly in the liver and bones.
Hazards of actinium
Extremely radioactive and radiotoxic: gram for gram, actinium-227 is more than a thousand times more radioactive than plutonium-239. It is handled only in shielded glove boxes.
Cosmic origin
- None of it survives from the formation of the Solar System: it is constantly being created by the radioactive decay of uranium and thorium.
History of actinium
André-Louis Debierne, a colleague of the Curies, identified it in 1899 in pitchblende residues. The German Friedrich Giesel discovered it independently in 1902 under the name “emanium.” Debierne's name, from the Greek aktis (“ray”), was the one that stuck.
Did you know?
- It lent its name to 15 elements, from actinium to lawrencium.
- A single dose of actinium-225 against cancer contains just a few nanograms of material — a billionth of a gram each.
- Its blue glow comes not from the metal itself but from the air it ionizes around it.
Isotopes of actinium
No stable isotopes; actinium-227 forms in traces in uranium ores. Actinium-225 is one of nuclear medicine's great hopes against cancer.
- actinium-225: radioactive, half-life 9.92 days. Used in targeted alpha therapy: attached to a molecule, it destroys cancer cells at close range.
- actinium-227: radioactive, half-life 21.8 years. The longest-lived; used to produce the radium-223 in the drug Xofigo.
- actinium-228: radioactive, half-life 6.15 h
Related elements
Same family (actinides): Thorium (Th), Protactinium (Pa), Uranium (U), Neptunium (Np), Plutonium (Pu), Americium (Am), Curium (Cm), Berkelium (Bk), Californium (Cf), Einsteinium (Es), Fermium (Fm), Mendelevium (Md), Nobelium (No), Lawrencium (Lr).
Same column (group 3): Scandium (Sc), Yttrium (Y), Lanthanum (La).