Where/how do rare earths/actinides fit into the PERIODIC TABLE?

1751

Swedish Mineralogist A.F. Cronstedt discovers a new heavy mineral

1789

Klaproth shows pitchblende contains a new element, uranium

• isolated as a metal in 1841 by Peligot

1794

J. Gadolin isolates "yttria",

• but thought it was the pure oxide of Y

1794-1907

ca. 100 claims of elements in the rare earth group, due to:-

• problems of separation (very similar properties)

• lack of conclusive tests as to whether a mixture was involved

pre-1905

Mendeleevian Periodic table could only accommodate 1 element for this group, La

• at one stage W. Crookes thought that apparently different rare earths were "simply different modifications of the same element."

• Alfred Werner produced a Periodic table in 1905 of essentially current form, (though he did put spaces for 15 elements between La & Hf - there was no viable atomic theory at that time!)

1913

X-ray spectroscopy

• W. Moseley shows from characteristic X-ray patterns conclusively that there are only 14 elements between La & Hf

1918-1921

Bohr interprets this as expansion of 4th quantum group from 18 to 32 e^-

• Lanthanides are therefore identified as the first f-series

1939-45

Lanthanides recognised in fission products of uranium

• Techniques to obtain lanthanide separation greatly improved

pre-1940

of the actinides only U(1789), Th (1829), Ac(1889), Pa (1913) are known

post -1940

The nuclear age ~ uranium and plutonium chemistry

1940-1961

Trans-uranium elements synthesized by "bombardment" expts.

• Actinides developed as the 5f series equivalent of the lanthanides (Seaborg's Actinide Concept), & proven not to be a 4th transition series

post-1955

Lanthanides obtained in increasing amounts ~ increasing technological importance