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The actinoid (according to IUPAC terminology) (previously actinide) series encompasses the 15 chemical elements that lie between actinium and lawrencium included on the periodic table, with atomic numbers 89 - 103. The actinoid series derives its name from the first element in the series, actinium, and ultimately from the Greek aktis, "ray," reflecting the elements' radioactivity.

The actinoid series (An) 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 (moreover, from Latin, the suffix -ide means "sons of actinium", while -oid means "similar to actinium"). There are alternative arrangements of the periodic table that exclude actinium or lawrencium from appearing together with the other actinoids.

The actinoids display less similarity in their chemical properties than the lanthanoid series (Ln), 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 actinoids 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 actinoids are typically placed below the main body of the periodic table (below the lanthanoid series), in the manner of a footnote. The full-width version of the periodic table shows the position of the actinoids more clearly.

An organometallic compound of an actinoid is known as an organoactinoid.

From the earlier known chemical properties of actinium (89) up to uranium (92), indicating a relation to the transition metals, it was generally assumed that the transuraniums would have similar qualities. 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 than the transition metals, 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 lanthanoid elements.

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

Actinide element symbol

Chemical series of the periodic table

Alkali metals Alkali metals
Alkaline earth metals Alkaline earth metals
Lanthanides Lanthanides
Actinides Actinides
Transition metals Transition metals
Poor metals Poor metals
Metalloids Metalloids
Nonmetals Nonmetals
Halogens Halogens
Noble gases Noble gases

The periodic table of the chemical elements is a tabular method of displaying the chemical elements. Although precursors to this table exist, its invention is generally credited to Russian chemist Dmitri Mendeleev in 1869. Mendeleev intended the table to illustrate recurring ('periodic') trends in the properties of the elements. The layout of the table has been refined and extended over time, as new elements have been discovered, and new theoretical models have been developed to explain chemical behavior.

The periodic table is now ubiquitous within the academic discipline of chemistry, providing an extremely useful framework to classify, systematize and compare all the many different forms of chemical behavior. The table has also found wide application in physics, biology, engineering, and industry. The current standard table contains 117 confirmed elements as of October 16, 2006 (while element 118 has been synthesized, element 117 has not).

  1. Arrangement
  2. Periodicity of chemical properties
  3. Structure of the periodic table
  4. History of the periodic table
  5. A list of who discovered each element


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