1.    The Aufbau principle cannot be used to predict electron configuration of atoms on ionization. In other words, it does not tell us which electrons are to be removed when an ion is formed form an atom. For example, the configuration of Fe based on Aufbau principle is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d6. It has been experimentally confirmed by spectral and magnetic studies than tFe2+ has configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ and not 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁴. It means that ionization results in the loss of 4s electrons in preference to 3d electron even thought the 3d were the last to be added in building up the configuration of Fe atom. It means that in Fe2⁺ ,3d has lower energy than 4s which is contrary to Aufbau order of filling. This can be said of any transition element on ionization.

2.    Although (n-1) d subshells and ns subshells lie quite close together , yet the former is somewhat higher in energy. Accordingly the predicted Aufbau outer configurations of Cr (Z = 34) is .....3d⁴ 5s² but the experimentally supported configuration is  ....3d⁵ 4s¹.

3.    In the sixth period , 4f and 5d subshells are exceedingly close in energy . At lanthanum (Z = 57),the last electron does not go to 4f as predicted in Aufbau order but is added to 5d subshell to give it a configuration ....4d¹⁰ 5s³ 5p⁶ 5d¹ 6s³. However, the next electron for valium (Z =58) is added to the 4f subshell and the previous electron in 5d in lanthanum is also shifted to 4f to give it a configuration ....4d¹⁰ 5s² 5d⁶ 4f² 5d⁰ 6s².

4.    The Aufbau order configuration of zirconium (Z =40) is ....5s² 4d² and that the predicted order for the next element niobium Nb (Z =41) would be ...5s² 4d³ . However, the observed configuration of Nb is... 5s¹ 4d4. It is understandable that the next  element molybdenum Mo (Z =42) acquires.....5s¹ 4d5 configuration due to higher stability associated with half-filled configuration. However, the next element technetium Tc (Z =43) with observed configuration 5s¹ 4d6 in place of predicated 5s² 4d6 shows that the preference for half-filled or completely-filled configuration is not the only controlling factor but some complex nucleus=electron and electron-electron forces also have some role to plat in deterring these configuration.

5.    In case of palladium pd (Z=46), the predicted Aufbau configuration is 5s² 4d⁸ but the experimentally observed configuration is 5s⁰ 4d¹⁰. This is perhaps the only case where the observed configuration has two misplaced electron.

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