Question

The electronic configuration of actinoids cannot be assigned with degree of certainty because of

• A. small energy difference between $5f$ and $6d$ levels
• B. overlapping of inner orbitals
• C. free movement of electrons over all the orbitals
• D. none of the above.

Answer 1

Answer: (A)

For the first four actinide elements, $Th,Pa,U$ and Np, the difference in energy between $5f$ and $6d$ -orbitals is small. Thus, in these elements (and their ions) electrons may occupy the $5f$ or the $6d$ levels or sometimes both. Later in the actinide series the $5f$ -orbitals do become appreciably lower in energy. Thus, from Pu onwards the $5f$ -shell fills in a regular way and the elements become very similar.