These facts render their study
more difficult All the actinoids are believed to have the electronic configuration of 7s
2
and variable occupancy of the 5f and 6d subshells The fourteen electrons
are formally added to 5f, though not in thorium (Z = 90) but from Pa
onwards the 5f orbitals are complete at element 103 The irregularities in
the electronic configurations of the actinoids, like those in the lanthanoids
are related to the stabilities of the f 
0, f 
7 and f 
14 occupancies of the 5f
orbitals