The Lanthanides
The lanthanide series include the 14 elements that proceed lanthanum (atomic number 57) from atomic numbers 58 to 71.
Their electron configuration include the 4f and 5d energy levels.
Because of the closeness of those two levels, there is considerable uncertainty in some electron configuration assignments.
All the lanthanides form +3 ions as their principal chemical species.
It is assumed that the ions are formed by losing the 6s2 and 5d1 (or 4f is 5d is not present) electrons.
They generally occur together, except for promethium, which has an unstable nucleus.
The richest source mineral is monazite, a complex phosphate.
They are very rare to find, hence their nickname--"the rare earth elements."
Since they also have very similar chemical properties, separation is very difficult, involving fractional crystallization and ion-exchange techniques.
The lanthanides also generally have an incomplete 4f subshell, resulting in paramagnetism.
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The Actinides
The actinide series include the 14 elements that proceed actinium (atomic number 89) from atomic numbers 90 to 103.
The electron configurations of the actinides are even more uncertain than the lanthanides
because the closeness of the energy levels and because the nuclei are
unstable to radioactive decay.
Only minute amounts of some elements are obtained because of their
instability.
All of the actinides are unstable with respect to alpha emission.
The later members tend to undergo spontaneous fission, a fact which limits the number of elements possible.
The actinides also seem to show a variety of oxidation states, unlike the lanthanides.
Uranium, for example, has compounds in each of the states, +3, +4, +5, and +6.
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