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

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

The d-and f-Block Elements

Solution:

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