Example 1:  How is Iron(III) like Plutonium(IV)?

In designing sequestering agents for waste cleanup or decorporation agents for accidental ingestion of actinides, researchers often turn to ‘model’ metal cations to avoid the costs and risks associated with using actinide elements.  Lanthanides are sometimes used as actinide mimics, and are well suited for the later elements in the row (compare Nd(III) to Am(III) or Cm(III)). However, the early actinides are multifaceted elements with many fine points in their chemistry. The biological activity of Pu(IV) in mimicking Fe(III) pointed to similar characteristics in these two cations, which can be highlighted by comparison of their ‘charge to radius ratio’. Compare:

While oxidation state and ionic radius are different for Ce(IV), Fe(III), and Pu(IV), their charge to radius ratios (a rough surface charge measure) are similar. Ce(IV) is the best non-radioactive model for Pu(IV), as might be expected. While models have their use, ultimately the actinides should be studied directly.

Exercise:Another better guide might be to compare charge to surface area (A = 4 p r²); this is readily done using the spreadsheet. A) Set up an extra column on the spreadsheet, and enter the equation, Z/A (Z=charge, A=area (see above)). B) Compare charge to surface ratios of Pu(IV) to Fe(III), to Ce(IV)[coordination numbers 6 and 8, or others]. What is the result? Which cations are most alike? Different?

Example 2: Can the ionic radius of Pb(II) be linked to its toxicity?