Actinide

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The actinide (or actinoid) series encompasses the 15 chemical elements that lie between actinium and lawrencium on the periodic table, with atomic numbers 89 - 103^[1]. The actinide series is named after actinium.

The actinide series is included in some definitions of the rare earth elements. IUPAC is currently recommending the name actinoid rather than actinide, as the suffix "-ide" generally indicates ions. There are alternative arrangements of the periodic table that exclude actinium or lawrencium from appearing together with the other actinides.

The actinides display less similarity in their chemical properties than the lanthanide series, exhibiting a wider range of oxidation states, which initially led to confusion as to whether actinium, thorium, and uranium should be considered d-block elements. All actinides are radioactive.

Only thorium and uranium occur naturally in the earth's crust in anything more than trace quantities. Neptunium and plutonium have been known to show up naturally in trace amounts in uranium ores as a result of decay or bombardment. The remaining actinides were discovered in nuclear fallout, or were synthesized in particle colliders. The latter half of the series possess exceedingly short half-lives.

The actinides are typically placed below the main body of the periodic table (below the lanthanide series), in the manner of a footnote. The full-width version of the periodic table shows the position of the actinides more clearly.

An organometallic compound of an actinide is known as an organoactinide.

IUPAC currently recommends the name actinoid rather than actinide, as the suffix "-ide" generally indicates negative ions whereas the suffix "-oid" indicates similarity to one of the members of the containing family of elements.

[edit] History of the actinide series

During his Manhattan Project research in 1944, Glenn T. Seaborg experienced unexpected difficulty isolating Americium (95) and Curium (96). He began wondering if these elements more properly belonged to a different series which would explain why the expected chemical properties of the new elements were different. In 1945, he went against the advice of colleagues and proposed the most significant change to Mendeleev's periodic table to have been accepted universally by the scientific community: the actinide series.

In 1945, Seaborg published his actinide concept of heavy element electronic structure, predicting that the actinides would form a transition series analogous to the rare earth series of lanthanide elements.

In 1961, Antoni Przybylski discovered a star that contained unusually high amounts of actinides.