Actinium (Ac)

• Element Symbol: Ac
• Atomic Number: 89
• Atomic Mass: 227
• Group # in Periodic Table: n/a
• Group Name: Actinides
• Period in Periodic Table: 7
• Block of Periodic Table: f-block (disputed)
• Discovered by: André-Louis Debierne (1899)

Actinium is a naturally occurring radioactive element with the chemical symbol Ac. Its atomic number is 89, and its relative atomic weight is 227. Its electronic configuration is [Rn] 6d¹7s². In the periodic table, it is placed in Group 3b, within period 7, and it falls between the elements radium and thorium. Actinium is placed in a group of elements called the actinides. These elements are assigned atomic numbers from 89 to 103, and their chemical properties are similar to each other, but they differ from other elements in the periodic table.

In 1899, French chemist André-Louis Debierne discovered actinium when he was experimenting with novel methods for separating rare earth oxides from pitchblende. In 1902, Friedrich Otto Giesel also discovered actinium independently.

Actinium is a rare element found in nature within uranium ores. It exists in very tiny amounts in these ores. Nonetheless, it is easier and much less expensive to create actinium in a laboratory than by trying to extract it from the earth’s crust. It can be created by bombarding the element radium-88 with neutrons in a machine called nuclear reactor.

Debierne named actinium after the Greek word aktinos, which means "ray" or "beam." It is aptly named because it produces a subtle blue radiance, making it luminescent. This luminescence is due to the presence of a naturally occurring mineral known as pitchblende. This ore contains not only actinium, but also radium, radon, and polonium. A ton of pitchblende ore contains about 0.15 mg of actinium. Another name for actinium is emanium, also due to the glow it radiates.

Actinium is an important element in history insofar as it is one of the primordial elements. A primordial element is one that is said to have existed even before Earth was formed. These elements have exceptionally long half-lives.

Physical Properties

The standard state of an element is defined as its state at 298 (kelvin) K. Actinium is a solid at this temperature. It is a silvery-white metal that glows blue in the dark, and it has characteristics that are similar to those of the element lanthanum. Actinium has a face-centred cubic structure. Its boiling point is 3471 K, which translates to 3198°C or 5788°F, and its melting point is 1324 K (1051°C or 1924°F). Its density is 10.07 grams per cubic centimeter. Earth’s crust contains 5.5 × 10^-10 milligrams per kilogram of this element. Its ionization potential is 5.17 electron volts (eV), and it occurs in a +3 oxidation state. The ions of actinium are colorless when made into a solution.

Chemical Properties

Actinium is a naturally occurring trace element found in Earth’s crust. Its most stable isotope is actinium-227. This isotope has the longest half-life of all of the actinium isotopes—21,773 years. All other isotopes of actinium have half-lives that are much shorter—from less than ten hours to less than one minute. Actinium-227 undergoes alpha decay and turns into francium-223, or it undergoes beta decay and turns into thorium-227. Most of the actinium-227 decays into thorium-227, and only about 1 percent of it decays to francium-223. This disintegration chain is called the actinium series.

The first ionization energy for actinium is 499 kilojoules per mole (kJ/mol), and the second ionization energy is 1170 kJ/mol. Its specific heat capacity is 27.2 joules per mole Kelvin (J/mol K). The heat of fusion for actinium is 14.2 kJ/mol, whereas its heat of vaporization is 400 kJ/mol.

Actinium has thirty-six isotopes. Some exist naturally, whereas some are artificially produced. Actinium-225 decays through alpha decay and has a half-life of about ten days. The daughter isotopes of actinium-225 emit alpha and beta particles without any high-energy gamma rays.

Alpha decay is a process of radioactive decay in which an unstable nucleus loses two neutrons and two protons and becomes a more stable nucleus. This combination of two neutrons and two protons is called an alpha particle. Beta decay is a different radioactive-decay process, one in which the unstable nucleus releases an electron along with an unusual particle. This particle is called an antineutrino. The electron that is released from the unstable radioactive nucleus is called a beta particle. The electron is given the name beta particle so that it can be differentiated from the other electrons that orbit around the nucleus. Alpha and beta decays occur in radioactive elements that are neutron rich. In contrast to the alpha particle, a beta particle has a single negative charge and weighs less than an alpha particle. This characteristic makes the beta particle less reactive than the alpha particle.

Applications

Actinium, which has been widely been researched, is frequently used in the medical field—particularly in the field of oncology, where it has produced notable results. Isoptope-225 produces high-energy radiation. This radiation attacks a tumor without significantly affecting the neighboring tissue. The alpha particle emitted by actinium-225 has four beneficial properties that make it a drug of choice for treating cancer patients. It has a limited range of only a few cell diameters, thus reducing unnecessary damage to surrounding healthy tissue. It has high linear energy and a ten-day half-life, and it produces only four net alpha particles per decay. Furthermore, isotope-227 is about 150 times more radioactive than radium, making it a valuable source of neutrons.

A study revealed that alpha particle–emitting elements such as actinium have potential uses in radioimmunotherapeutic applications. This benefit results from the fact that these radioactive products are toxic to the cancerous tumor cells and nontoxic to noncancerous cells. Actinium-225 has been studied as a prospective target drug that can be used to treat acute myeloid leukemia (AML).

Actinium Pharmaceuticals has developed a drug called Actimab-A that is being studied as a treatment for AML. This company studied patients over the age of sixty who did not receive previous treatment and who had relapsed or were contraindicated for rigorous induction chemotherapy. A total of fourteen patients, those who had an average cluster of differentiation (CD3) expression of 81 percent, were divided into four groups. One group was treated with 0.5 microcuries per kilogram (µCi/kg) of the drug. Patients in each one of the other three groups was treated with rising levels of the drug: 1.0 µCi/kg, 1.5 µCi/kg, and 2.0 µCi/kg. After the first round of treatment, 72 percent bone marrow reduction was seen in eight out of eleven patients, which translates to 73 percent of the patients. Four out of fourteen patients, meaning 29 percent of the patients, showed a fair response; one of those patients showed complete remission; two patients attained complete remission, with incomplete platelet count retrieval; and one patient had complete remission, with incomplete blood count recuperation.

Bibliography

"Actinium Receives Orphan-Drug Designation From FDA for Actimab-A in the Treatment of Newly Diagnosed Acute Myeloid Leukemia in Elderly Patients." Actinium Pharmaceuticals. Market Wired, 1 Dec. 2014. Web. 1 Feb. 2016.

"Alpha Decay." Glossary. The Jefferson Lab, n.d. Web. 29 Jan. 2016.

"Beta Particle Radiation." The Beta Particle. Idaho State University, n.d. Web. 1 Feb. 2016.

"Chemistry in Its Element—Actinium." Royal Society of Chemistry. Royal Society of Chemistry, n.d. Web. 30 Jan. 2016.

"The Element Actinium." It’s Elemental. The Jefferson Lab, n.d. Web. 30 Jan. 2016.

Morss, Lester. "Actinium." Encyclopaedia Britannica. Encyclopedia Britannica Inc., n.d. Web. 1 Feb. 2016.