In actinoids, 5f and 6d subshells are close in energy. The outermost 7s orbital remains filled with 2 electrons (7s2). The electron can easily jump from 5f to 6d or vice versa. Further, irregularities in electronic configurations are also related to the stabilities of f0 , f7 and f14 occupancy of the 5f-orbitals. Hence, they show a large number of oxidation states (Moreover, they are radioactive with short half-lives. Hence, their properties cannot be studied easily).
