Actinium is the third element in Row 7 of the periodic table, a chart that shows how the chemical elements are related to each other. Some chemists place it in Group 3 (IIIB), with scandium and yttrium. Other chemists call it the first member of the actinides. The actinides are the 14 elements that make up Row 7 of the periodic table. They have atomic numbers from 89 to 103 and are all radioactive. A radioactive atom is unstable and tends to throw off particles and emit energy in order to become stable. Either way of classifying actinium is acceptable to most chemists.
Actinium has chemical properties like those of lanthanum (number 57), the element just above it in the periodic table. Actinium is also similar to radium, the element just before it (number 88) in Row 7.
Naturally occurring actinium is very rare in the Earth's crust. It can be made in the lab by firing neutrons at radium, but it has very few important uses.
Group 3 (IIIB)
Discovery and naming
Four new elements, all radioactive, were discovered between 1898 and 1900. A radioactive element is one that gives off radiation in the form of energy or particles and may change into a different element. The first two of these elements— polonium and radium—were discovered by Marie Curie (1867-1934) and Pierre Curie (1859-1906). The third, actinium, was discovered in 1899 by a close friend of the Curies, French chemist André Debierne (1874-1949). Debierne suggested the name actinium for the new element. The name comes from the Greek words aktis or aktinos, meaning "beam" or "ray." The fourth element discovered in this series was radon, a gas given off during the radioactive decay of some heavier elements. It was found in 1900 by German chemist Friedrich Ernst Dorn (1848-1916).
Actinium was discovered a second time in 1902. German chemist Friedrich 0. Giesel (1852-1927) had not heard of Debierne's earlier discovery. Giesel suggested the name emanium, from the word emanation, which means "to give off rays." Debierne's name was adopted, however, because he discovered actinium first.
Physical and chemical properties
Only limited information is available about actinium. It is known to be a silver metal with a melting point of 1,050°C (1,920°F) and a boiling point estimated to be about 3,200°C (5,800°F). The element has properties similar to those of lanthanum. Generally speaking, elements in the same column in the periodic table have similar properties. Few compounds of actinium have been produced. Neither the element nor its compounds have any important uses.
Occurrence in nature
Actinium is found in uranium ores. An ore is a mineral mined for the elements it contains. It is produced by the radioactive decay, or breakdown, of uranium and other unstable elements. Actinium can also be artificially produced. When radium is bombarded with neutrons, some of the neutrons become part of the nucleus. This increases the atomic weight and the instability of the radium atom. The unstable radium decays, gives off radiation, and changes to actinium. Actinium metal of 98 percent purity—used for research purposes—can be made by this process.
About a dozen isotopes of actinium are known. All are radioactive. The two that occur in nature are actinium-227 and actinium-228. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope. A radioactive isotope is one that breaks apart and gives off some form of radiation.
The half lives of actinium-227 and actinium-228 are 21.77 years and 6.13 hours, respectively. The half life of a radioactive element is the time it takes for half of a sample of the element to break down. For example, suppose 1.0 gram of actinium-227 is formed by the breakdown of another element. After 21.77 years, only 0.5 gram of actinium-227 would remain. This is known as the half life.
Actinium is rarely, if ever, extracted from natural sources.
There are no practical commercial uses of actinium. Actinium of 98 percent purity is prepared for research studies.
The few compounds of actinium that are known are used solely for research purposes.
Like all radioactive materials, actinium is a health hazard.
Like all radioactive materials, actinium is a health hazard. If taken into the body, it tends to be deposited in the bones, where the energy it emits damages or destroys cells. Radiation is known to cause bone cancer and other disorders.