Actinium

Actinium is the first non-primordial radioactive element to be isolated. That means it’s the first radioactive element discovered that has only existed since after the Earth was formed.

First discovered in 1899 by French chemist Andre-Louis Debierne, actinium was named after the Greek word aktinos, meaning ray or beam, because of its bluish glow. Despite the discovery, he later lost interest in the element and the torch was taken up by Friedrich Oskar Giesel, a German organic chemist. Giesel originally thought he discovered another new element in 1902, but after comparing the half-lives of his material and actinium, it became apparent that he had become the first person to isolate the element.

Giesel continued his research on actinium and produced the first pure sample of the element. That’s why he gets credit for identifying its atomic number.

Actinium is the first element of the actinides and gave the set of 15 elements its name. It is a soft, silver-colored metallic element that sometimes has a golden cast. Yes, it glows in the dark with an eerie blue light. The blue color comes from the radioactive ionization of gases in the air.

In metal form, it is a good conductor of both heat and electricity. It has chemical properties that are similar to lanthanum, the element located directly above actinium in the periodic table. It has a density of 10.07 grams per cubic centimeter. Actinium’s melting point is 1050.0°C and the boiling point is 3200.0°C. It tarnishes easily in the air, forming a white coating of actinium oxide layer.

Right now, actinium doesn’t have a lot of uses. It’s used primarily in radiological research because it is a very powerful source of alpha rays and a good neutron source. Actinium-225 shows promising results as a radiation therapy to treat cancer.

The stable isotope actinium-277 has a half-life of 21.77 years, which makes it useful in modeling the slow, vertical mixing of oceanic waters. Researchers evaluate the concentration depth-profiles for actinium isotopes, which allows them to estimate the mixing rates and model the mixing behavior in the deep ocean. It also may be used for thermoelectric generators in spacecraft.

Actinium is intensely radioactive, so it has a limited number of known compounds. These compounds include actinium fluoride (AcF₃₎, actinium chloride (AcCl₃), actinium bromide (AcBr₃), actinium oxyfluoride (AcOF), actinium oxychloride (AcOCl), actinium oxybromide (AcOBr), actinium sulfide (Ac₂S₃), actinium oxide (Ac₂O₃), actinium phosphate (AcPO₄), and actinium nitrate Ac(NO₃)₃. They all contain actinium in the oxidation state.

Actinium is observed to form oxides, halides, oxyhalides, and oxophosphates. All isotopes of actinium are radioactive. Naturally occurring actinium is composed of two radioactive isotopes, actinium-227 (from uranium-235) and actinium-228 (from thorium-232).

Thirty-six radioisotopes have been identified, with the most stable being actinium-227 (half-life 21.772 years), actinium-225 (half-life 10 days), and actinium-22c (half-life 29.37 hours). All the remaining radioisotopes have half-lives shorter than 10 hours, with most being shorter than one minute.

The shortest-lived known radioisotope is actinium-217 (half-life 69 nanoseconds).

Even though actinium is a rare element, it is found naturally in uranium ores, where it forms from radioactive decay of uranium. Actinium has a very small natural concentration, which makes separation of the element difficult and impractical. In fact, complete separation of actinium from uranium can never be achieved!

To get pure actinium, researchers bombard radium with neutron radiation in a nuclear reactor. However, only small amounts—meaning milligrams—can be made. After the actinium is synthesized, scientists can separate the actinium from any remaining radium or contaminants with a chemical solution and mild acid. Scientists can also separate actinium with resins and nitric acid.