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 Thus, the configurations of Am and Cm are [Rn] 5f 
77s
2 and
[Rn] 5f
 76d
17s
2 Although the 5f orbitals resemble the 4f orbitals in their
angular part of the wave-function, they are not as buried as 4f orbitals
and hence 5f electrons can participate in bonding to a far greater extent