Atomic Sizes of Actinides

name         Heather
status       student
grade        9-12
location     IA

Question -   What are the trends in atomic size for the actinides?
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I do not believe that there are any "simple" trends like the trends 
found and "explained" for lighter elements. There are several 
reasons for this. First, there are several measures of "atomic 
size" which may or may not apply to the actinides. There are 
Pauling radii, ionic radii (that depend on the oxidation number of 
the ion), Allred radii, metallic radii (which are determined from 
the crystal structure of the metal). Second, the fundamental 
difficulty lies with the fact that the atomic shells being filled 
in the actinide series are: 7s, 6d, 5f and the order of filling 
which shells "first" changes from element to element because these 
states are very close in energy. This electronic ambivalence is 
reflected in the fact the actinides typically have 3-5 valence 
states depending upon the compound, and the elemental metals have 
several crystal modifications --
the electrons are jumping from one configuration to another.
    Devising trends in atomic properties such as atomic size only 
works well when the electronic states involved differ significantly 
in their energy, and even then one needs to be cautious in not 
"over explaining" some trend that is only accidental, or even 
wrong. For example, consider some of the lighter elements: Why 
should oxygen be blue? Or the halogens vary in color from yellow 
(F2) to purple (I2) when a simple "dot" structure would appear to 
be satisfied by the octet rule?

Vince Calder
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