The electron configurations of transition elements involved exceptions to the expected filling order in both the d and f subshells. In the filling of the first transition series, chromium has one more d (one less s electron) than expected. The left graph shows the ionization energy of the first transition series. There are two significant jumps in ionization energy, one from chromium to manganese and the other from copper to zinc. Generally a large change in IE would indicate the change to a filled or half filled subshell.

The large increase from copper to zinc is interpreted in terms of both the 3d and 4s being filled. The increase from chromium to manganese suggests that half filled subshells have a greater degree of stability than subshells whose orbitals have other filling patterns.

For the second transition series, the changes are more uniform until palladium is reached. This suggests that the electron configurations of the 4d elements vary from those in the 3d electrons. The second row transition elements display electron configurations with many deviations in filling order. For example, palladium is shown to have the 4d subshell filled and the 5s empty This would indicate it should be difficult to remove an electron from palladium and the IE shows that.