An actinoid that does not have one electron in its 6d orbital is -

1. U (Atomic number. 92) 2. Np (Atomic number. 93)
3. Pu (Atomic number. 94) 4. Cm (Atomic number. 96)

HINT: All elements belong to actinoids.Pu does not have electron in d- orbitals.


The electronic configuration of U, Np, Pu, and Cm are as follows:

U=92, Rn5f3 6d17s2
Np=93, Rn 5f46d1 7s2
Pu=94,Rn 5f6 7s2
Cm=96, Rn 5f7 6d17s2

Thus, Pu does not have electron in 6d orbital.

Hence, option third is the correct answer.