Actinium – facts and properties
- Symbol
- Ac
- Atomic number
- 89
- Atomic mass
- 227 u
- Period
- 7
- Category
- actinide
- Electron configuration
- [Rn] 6d¹ 7s²
- Electronegativity
- 1.1
- Atomic radius
- 195 pm
- Valence electrons
- 3
- State at standard conditions
- solid
- Melting point
- 1051 °C
- Boiling point
- 3198 °C
- Density
- 10.07 g/cm³
- Discovered
- 1899
Actinium is a silvery-white, strongly radioactive metal with atomic number 89. It opens the series of the actinoids, which is named after it, and sits below lanthanum in the table. André-Louis Debierne discovered it in 1899 in the residues of pitchblende. Its radiation is so intense that it shimmers blue in the dark. Medically interesting is the isotope actinium-225, an alpha emitter for new kinds of cancer therapy.
Position in the periodic table
Occurrence and extraction
Actinium is one of the rarest elements in nature. It forms in the uranium-235 decay series, and its longest-lived isotope, actinium-227, has a half-life of about 22 years. A tonne of uranium in an ore holds only a fraction of a milligram of it. It cannot be extracted from ores economically, because the effort would be out of proportion to the yield.
Debierne, a colleague and friend of the Curies, found a new radioactive substance in pitchblende residues in 1899. It behaved chemically like lanthanum, which made separating it difficult. Independently, Friedrich Giesel described an element in Germany in 1902 that he called emanium. It was the same one, and the name actinium prevailed. It comes from the Greek aktis, meaning ray.
Today actinium-227 is made artificially: radium-226 is bombarded with neutrons in a nuclear reactor, changes via radium-227 into actinium-227, and is then separated chemically. Only small amounts result, and they are handled in specialised laboratories.
Properties and reactions
Actinium is in period 7 and is the first actinoid. The table usually shows the actinoids as a separate row below the main body, and actinium stands there under lanthanum, with thorium to its right. It has the electron configuration [Rn] 6d¹ 7s² and an electronegativity of 1.1, and its atomic radius is 195 pm. Its density is 10.07 g/cm³; it melts at 1051 °C and boils only at 3198 °C. Its atomic mass is given as 227 u, the mass number of actinium-227.
Chemically, actinium is very similar to lanthanum. It is a soft base metal that releases hydrogen from water and coats itself with an oxide layer in air. In compounds it appears almost only as a triply charged positive ion. Because of this resemblance it was long hard to tell apart from lanthanum and the lanthanoids. With it begins the series of elements in which the 5f subshell is filled, although actinium itself has no 5f electron yet.
The radiation defines the element. Actinium-227 emits beta and alpha radiation and decays through a chain of further radionuclides. Per gram it radiates far more strongly than radium. The surrounding air is excited in the process, so the metal shimmers blue in the dark. In the body, actinium settles mainly in bone and liver, where it irradiates the tissue for years. Only trained specialists may handle it, under strict conditions.
Research and synthesis
Outside the laboratory and the clinic, actinium has no application. In research, actinium-227 together with beryllium served as a neutron source, because alpha particles knock neutrons out of beryllium. It is also used as a tracer for studying how the oceans mix.
Much attention goes to the isotope actinium-225, with a half-life of about ten days. In a single decay chain it emits several alpha particles. Alpha particles release their energy within a few cell diameters, which is an advantage for treating tumours. In alpha radioligand therapy the isotope is attached to a molecule that binds specifically to cancer cells, for example to a marker of prostate cancer. The radiation then destroys mainly the diseased cells and largely spares the surrounding tissue. These methods are mostly still being tested in clinical trials.
The difficulty is supply. Actinium-225 is obtained from the decay of thorium-229, which comes from old stocks of uranium-233, or it is produced in particle accelerators. The available amounts are small and so far limit how many patients can be treated.
Quick question
What is the atomic number of actinium (Ac)?
FAQ
Why does actinium glow blue in the dark?
The strong radiation of actinium excites the air around the metal, and the excited particles give off bluish light. The glow is a sign of the radiation. It is a hazard, not a spectacle.
What is actinium-225 used for?
As an alpha emitter in cancer research. In alpha radioligand therapy a carrier molecule brings the isotope to tumour cells, and its short-range radiation destroys mainly those cells. The use is still being examined in trials.
Where does the name actinium come from?
It goes back to the Greek word aktis, which means ray. The element gives off powerful radiation. Later the whole series of elements from actinium to lawrencium was named the actinoids after it.