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# Why can't the electron configuration of actinides be properly assigned with a high degree of certainty?

1) Overlapping of inner orbitals $\quad$ 2) Free movement of electrons across orbitals $\quad$ 3) Very small energy difference between 5f and 6d levels $\quad$ 4) Cannot be explained

Answer: Very small energy difference between 5f and 6d levels
The electrons in Actinids series and their ions may occupy the 5f or the 6d levels, or sometimes both. This is because of the very small energy difference between the 5f and 6d levels. Hence we cannot say with a high degree of confidence what the electron configuration is.