ACTINIUM

GREG WALL, SIGMA-ALDRICH

ACTINIUM AT A GLANCE
Name: From the Greek aktinos or aktis, ray or beam. Actinium is so intensely radioactive it glows in the dark.
Atomic mass: (227).
History: Discovered twice independently, first by the French scientist André Debierne in 1899 and then by the German Friedrich Otto (Fritz) Giesel in 1902.
Occurrence: Rare in nature; found in association with uranium minerals.
Appearance: Silvery white, solid metal. Chemically, it resembles the rare-earth element lanthanum.
Behavior: Highly radioactive but not toxic; reacts with water to give off hydrogen gas.
Uses: Limited commercial use (the annual world production is probably less than a gram). Because it is powerfully radioactive (150 times as active as radium), it is useful in producing neutrons.
I remember memorizing the periodic table for what was sure to be a test question. Although it wasn't, I learned a lot about the organization and structure of the periodic table and a group of elements known as the second inner transition series. This series starts with actinium and is characterized by the filling of the d-orbital valence shell. As chemists, we are exposed to the periodic table early in our careers, yet we only work with approximately one-fifth of the elements throughout most of our research. What about all of those other elements?

If the periodic table were the chemist's alphabet, we would naturally start with actinium, one of the rarest of all elements. I find it a very fascinating element. Here's why:

Lights, camera, ACTINIUM! The glow must go on. Glowing in the dark is a property shared by few elements. Actinium glows with an eerie blue light because its intense radioactivity excites the surrounding air, which in the process gets oxidized. Found in natural uranium ores, it has an estimated crustal abundance of 5.5 x 10­10 mg per kg.

Actinium, 89 on the periodic table, belongs to a group of elements also known as the rare-earth elements. It is a group 3 element and the first in a series of elements known as the actinides. Actinides are members of the transition metals exhibiting properties such as low ionization energies, positive oxidation states, high melting points, high boiling points, high electrical conductivity, and malleability. Its chemical behavior is similar to that of lanthanum, 57 on the periodic table and located just above actinium.

Actinium is a silvery white metal having a face-centered cubic crystal structure. It has an electronic configuration of [Rn] 6d17s2, a melting point of 1,320 K, a boiling point of 3,470 K, electronegativity of 1.1 (Pauling), and a thermal conductivity of 12 W m-1 K-1 at 300 K. There are 34 isotopes and isomers of actinium, and they are all radioactive. Actinium is approximately 150 times as active as radium, and it is used in the production of neutrons. It is primarily found as Ac3+ as either the oxide or hydroxide and as the halide salts. It is found in all uranium ores.

TOP MODEL The Fermi surface of actinium.
COURTESY OF TAT-SANG CHOY AND THE UNIVERSITY OF FLORIDA
Actinium was first isolated by André Debierne in 1899 from pitchblende residues left by the Curies after they had extracted radium. He reported his findings in a paper entitled "Sur la radioactivité induite provoquée par les sels d'actinium" in Comptes rendus hebdomadaires des séances de l'Académie des sciences [136, 446 (1903)]. Fritz Giesel observed emanations from "emanium" and reported on these findings in Chemische Berichte [36, 342 (1903)]. In 1906, it was concluded that Giesel's emanium was identical to Debierne's actinium. Over the next 20 years, the relationships among the members of the actinium decay series were determined by radiochemical studies, and the mass numbers of the members were established by Arthur Dempster in 1935 by spectrometric analysis. Today, actinium is prepared by bombarding radium with neutrons in a nuclear reactor or by the reduction of actinium fluoride with lithium vapor at 1,100 to 1,300 °C.

Actinium is primarily a -emitter. It is classified as a hazardous radioactive poison having high specific -activity along with radium and the transuranium elements plutonium, americium, and curium.

Upon exposure to actinium, the human body accumulates actinium in the superficial layers of the bone structure of the skeleton, where it is tenaciously retained. The rate of radioactive decay is greater than the rate at which it is eliminated. Actinium comes into equilibrium with its decay products in about 185 days and then decays, having a half-life of 21.8 years. It is about as dangerous as plutonium.

Currently, actinium has no significant commercial applications. It has been used in thermoelectric power generation and in the production of neutrons.


Greg Wall is a technical service scientist for Sigma-Aldrich. He is actively involved in promoting chemical literacy through public outreach programs, the newspaper, and radio and has been cited in the Wall Street Journal, Genetic Engineering News, and on BBC's "Nature's Magic."
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