Properties and Reactions of the Actinide Series of Elements

Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels.

At the bottom of the periodic table is a special group of metallic radioactive elements called actinides or actinoids. These elements, usually considered ranging from atomic number 89 to atomic number 103 on the periodic table, have interesting properties, and play a key role in nuclear chemistry.

Location

The modern periodic table has two rows of elements below the main body of the table. The actinides are the elements in the bottom of these two rows, while the top row is the lanthanide series. These two rows of elements are placed below the main table because they don't fit into the design without making the table confusing and very wide.

However, these two rows of elements are metals, sometimes considered a subset of the transition metals group. In fact, the lanthanides and actinides are sometimes called the inner transition metals, referring to their properties and position on the table.

Two ways of placing the lanthanides and actinides within a periodic table are including them in their corresponding rows with the transition metals, which makes the table wider, or ballooning them out, making a three-dimensional table.

Elements

There are 15 actinide elements. The electronic configurations of the actinides utilize the f sublevel, with the exception of lawrencium, a d-block element. Depending on your interpretation of the periodicity of the elements, the series begins with actinium or thorium, continuing to lawrencium. The usual list of elements in the actinide series is:

• Actinium (Ac)
• Thorium (Th)
• Protactinium (Pa)
• Uranium (U)
• Neptunium (Np)
• Plutonium (Pu)
• Americium (Am)
• Curium (Cm)
• Berkelium (Bk)
• Californium (Cf)
• Einsteinium (Es)
• Fermium (Fm)
• Mendelevium (Md)
• Nobelium (No)
• Lawrencium (Lr)

Abundance

The only two actinides found in appreciable quantities in the Earth's crust are thorium and uranium. Small quantities of plutonium and neptunium are present in uranium orders. Actinium and protactinium occur as decay products of certain thorium and uranium isotopes. The other actinides are considered synthetic elements. If they occur naturally, it is part of a decay scheme of a heavier element.

Common Properties

Actinides share the following properties:

• All are radioactive. These elements have no stable isotopes.
• Actinides are highly electropositive.
• The metals tarnish readily in air. These elements are pyrophoric (spontaneously ignite in the air), particularly as finely divided powders.
• Actinides are very dense metals with distinctive structures. Numerous allotropes can be formed—plutonium has at least six allotropes. The exception is actinium, which has fewer crystalline phases.
• They react with boiling water or dilute acid to release hydrogen gas.
• Actinide metals tend to be fairly soft. Some can be cut with a knife.
• These elements are malleable and ductile.
• All the actinides are paramagnetic.
• All these elements are silver-colored metals that are solid at room temperature and pressure.
• Actinides combine directly with most nonmetals.
• The actinides successively fill the 5f sublevel. Many actinide metals have properties of both d block and f block elements.
• Actinides display several valence states, typically more than the lanthanides. Most are prone to hybridization.
• The actinides (An) may be prepared by reduction of AnF3 or AnF4 with vapors of Li, Mg, Ca, or Ba at 1100-1400 C.

Uses

For the most part, we don't often encounter these radioactive elements in daily life. Americium is found in smoke detectors. Thorium is found in gas mantles. Actinium is used in scientific and medical research as a neutron source, indicator, and gamma source. Actinides may be used as dopants to make glass and crystals luminescent.

The bulk of actinide use goes to energy production and defense operations. The primary use of the actinide elements is as nuclear reactor fuel and in the production of nuclear weapons. The actinides are favored for these reactions because they readily undergo nuclear reactions, releasing incredible amounts of energy. If the conditions are right, the nuclear reactions can become chain reactions.

Sources

• Fermi, E. "Possible Production of Elements of Atomic Number Higher than 92." Nature, Vol. 133.
• Gray, Theodore . "The Elements: A Visual Exploration of Every Known Atom in the Universe." Black Dog & Leventhal.
• Greenwood, Norman N. and Earnshaw, Alan. "Chemistry of the Elements," 2nd edition. Butterworth-Heinemann.