Atomic No.	Name	Symbol
89	Actinium	Ac
90	Thorium	Th
91	Protactinium	Pa
92	Uranium	U
93	Neptunium	Np
94	Plutonium	Pu
95	Americium	Am
96	Curium	Cm
97	Berkelium	Bk
98	Californium	Cf
99	Einsteinium	Es
100	Fermium	Fm
101	Mendelevium	Md
102	Nobelium	No
103	Lawrencium	Lr

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 derives its name from the first element in the series, 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.

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.

See also

• Actinides in the environment

References

• The Columbia Encyclopedia, Sixth Edition.
• Chemical Elements website
• Actinides on the Book Rags website
• Lawrence Berkeley Laboratory image of historic periodic table by Seaborg showing actinide series for the first time
• Lawrence Livermore National Laboratory, Uncovering the Secrets of the Actinides
• Los Alamos National Laboratory, Actinide Research Quarterly

•  • [ e] Periodic tables
Layouts	Standard Vertical Full names Names and atomic masses Text for last Huge table Metals and nonmetals Blocks Valences Inline f-block 218 elements Electron configurations Atomic masses Electronegativities Alternatives
Lists of elements	Name Atomic symbol Atomic number Boiling point Melting point Density Atomic mass
Groups	1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Periods:	1 2 3 4 5 6 7 8
Series	Alkalis Alkaline earths Lanthanides Actinides Transition metals Poor metals Metalloids Nonmetals Halogens Noble gases
Blocks	s-block p-block d-block f-block g-block