Actinides (Actinide Series)

Properties and Reactions of the Actinide Series of Elements

The actinides are the elements in the bottom row of the periodic table, from actinium to lawrencium.
The actinides are the elements in the bottom row of the periodic table, from actinium to lawrencium. Alfred Pasieka / Getty Images

Actinides Definition

The actinides or actinoids are a set of radioactive elements on the periodic table, usually considered ranging from atomic number 89 to atomic number 103.

Location of the Actinides

The modern periodic table has two rows of elements below the main body of the table. The actinides are the elements in the bottom row. The top row is the lanthanide series. The reason these two rows of elements are placed below the main table is because they don't fit in 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 including the lanthanides and actinides within a periodic table are to include those elements in their corresponding rows with the transition metals (makes the table wider) or ballooning them out to make a three-dimensional table.

List of Elements in the Actinide Series

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
  • thorium
  • protactinium
  • uranium
  • neptunium
  • plutonium
  • americium
  • curium
  • berkelium
  • californium
  • einsteinium
  • fermium
  • mendelevium
  • nobelium
  • lawrencium

Actinide 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 of the Actinides

Actinides share the following common 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 air), particularly as finely divided powders.
  • Actinides are very dense metals with distinctive structures. Numerous allotropes may be formed (plutonium has at least 6 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 of the actinides are paramagnetic.
  • All of 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.