Exceptions
Although the Aufbau rule accurately predicts the electron configuration of most elements, there are notable exceptions among the transition metals and heavier elements. The reason these exceptions occur is that some elements are more stable with fewer electrons in some subshells and more electrons in others. A list of the exceptions to the Aufbau process is given below.
Period 4: Period 5:
Chromium: Z:24 [Ar] 3d54s1
Niobium: Z:41 [Kr] 5s1 4d4
Copper: Z:29 [Ar] 3d104s1
Molybdenum: Z:42 [Kr] 5s1 4d5
Ruthenium: Z:44 [Kr] 5s1 4d7
Rhodium: Z:45 [Kr] 5s1 4d8
Palladium: Z:46 [Kr] 4d10
Silver: Z:47 [Kr] 5s1 4d10
Period 6: Period 7:
Lanthanum: Z:57 [Xe] 6s2 5d1
Actinium: Z:89 [Rn] 7s2 6d1
Cerium: Z:58 [Xe] 6s2 4f1 5d1 Thorium: Z:90 [Rn] 7s2 6d2
Gadolinium: Z:64 [Xe] 6s2 4f7 5d1 Protactium: Z:91 [Rn] 7s2 5f2 6d1
Platinum: Z:78 [Xe] 6s1 4f14 5d9 Uranium: Z:92 [Rn] 7s2 5f3 6d1
Gold: Z:79 [Xe] 6s1 4f14 5d10
Neptunium: Z:93 [Rn] 7s2 5f4 6d1
Curium: Z:96 [Rn] 7s2 5f7 6d1
Lawrencium: Z:103 [Rn] 7s2 5f14 7p1
Table 1: Exceptions to Electron Configuration Trends