Introduction

Abstract

The actinide elements are the 15 chemical elements with atomic numbers 89 through 103, the first member of which is actinium and the last member is lawrencium (Fig. 1.1). The transactinide elements (those beyond the actinides) are the heaviest known chemical elements. Both the actinide and the transactinide elements have chemical properties that are governed by their outermost electronic subshells. Each of these groups of elements is a unique transition series (a group of elements in which d or f electronic subshells are being filled).

The actinides are the transition elements that fill the 5f subshell.The actinide series is unique in several respects:

• •Most of the elements (those heavier than uranium) were first discovered by synthetic methods: bombardment of heavy atoms with neutrons in nuclear reactors, bombardment with other particles in accelerators, or as the result of nuclear detonations.

• •All actinide isotopes are radioactive, with a wide range of nuclear properties, especially that of spontaneous and induced nuclear fission.

• •They are all metals with very large radii, and exist in chemical compounds and in solution as cations with very large ionic radii.

• •The metals exhibit an unusual range of physical properties. Plutonium, with six allotropes, is the most unusual of all metals.

• •Many of the actinide elements have a large number of oxidation states. In this respect plutonium is unique, being able to exist in aqueous solution simultaneously in four oxidation states.

• •In metallic materials and in some other compounds with elements lighter than plutonium, the 5f orbitals are sufficiently diffuse that the electrons in these orbitals are “itinerant” (delocalized, chemically bonding, often with unique magnetic moments and electrical conductivity). In metallic materials and in most compounds with elements heavier than plutonium the 5f electrons are “localized” (not contributing significantly to electrical conductivity or to chemical bonds). Materials with plutonium and adjacent elements can exhibit both itinerant and localized behavior, depending on conditions such as temperature and applied pressure.

• •Actinium (which has no 5f electrons in the metal, free atom, or in any of its ions) and the elements americium through lawrencium are similar in many respects to the lanthanide elements (the elements that fill the 4f electron subshell). The elements thorium through neptunium have some properties similar to those of the d transition elements.

• •Relativistic contributions to electronic properties and spin-orbit effects are important in the chemical properties of actinides.