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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

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The correct option of this question is (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 become appreciably lower in energy. Thus, from Pu onwards the 5f-shell fills in a regular way.

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