Final answer:
Exceptions to the Aufbau principle often occur with transition metals and some lanthanides and actinides, including curium (Cm), due to very similar energy levels and complex interactions that lead to variations in electron configurations.
Step-by-step explanation:
The Aufbau principle guides us through the process of adding electrons to atomic orbitals in the order of increasing atomic numbers. According to this principle, each electron occupies the lowest energy subshell available, obeying the Pauli exclusion principle. However, there are exceptions to this rule, particularly seen in transition metals like chromium (Cr) and copper (Cu), as well as in some instances within the lanthanides and actinides such as curium (Cm).
Speaking of curium (Cm), which has an atomic number of 96, its electron configuration does not strictly follow the predicted order. Normally, we would expect it to fill the 5f and 7s subshells systematically according to the Aufbau process. However, due to the closeness in energy levels and the complex interactions within the f-subshell, small changes can lead to a different order of filling. This leads to an actual electron configuration for curium that is slightly different from the predicted, with a few electrons placed in different subshells to achieve greater stability.
The exceptions in the periodic table often arise because subshells of very similar energy levels can be filled differently due to stabilizing effects that are not accounted for in the basic Aufbau approach. This is particularly true for elements with a higher atomic number where the electron-electron and electron-lattice interactions become more complex and significant.