Aufbau means "building up" in German, and the Aufbau principle states that electrons fill electron shells around atoms according to the energy level. This means that electron shells and subshells around atoms are filled from the inside out, except in some cases where an outer shell has a low energy level and partly fills up before an inner shell is full.
TL;DR (Too Long; Didn't Read)
Exceptions to the Aufbau principle are based on the fact that a few atoms are more stable when their electrons fill or half-fill an electron shell or subshell. According to the Aufbau principle, these electrons should always fill shells and subshells according to increasing energy levels. Elements such as copper and chromium are exceptions because their electrons fill and half-fill two subshells, with some electrons in the higher energy level shells.
Filling Electron Shells and Subshells
The electrons around an atomic nucleus have discrete energy levels called shells. The lowest energy level is closest to the nucleus, and it has room for only two electrons in a shell called the s shell. The next shell has room for eight electrons in two subshells, the s and the p subshells. The third shell has room for 18 electrons in three subshells, the s, p and d subshells. The fourth shell has four subshells, adding the f subshell. The lettered subshells always have room for the same number of electrons: two for the s subshell, six for p, 10 for d and 14 for f.
To identify a subshell, it is given the number of the main shell and the letter of the subshell. For example, hydrogen has its only electron in the 1s shell while oxygen, with eight electrons, has two in the 1s shell, two in the 2s subshell and four in the 2p subshell. The subshells fill up in the order of their numbers and letters up to the third shell.
The 3s and 3p subshells fill up with two and six electrons, but the next electrons go into the 4s subshell, not the 3d subshell as expected. The 4s subshell has a lower energy level than the 3d subshell and therefore fills up first. Although the numbers are out of sequence, they respect the Aufbau principle because the electron subshells fill up according to their energy levels.
How the Exceptions Work
The Aufbau principle holds for almost all elements, especially within the lower atomic numbers. Exceptions are based on the fact that half-full or full shells or subshells are more stable than partially filled ones. When the difference in energy levels between two subshells is small, an electron may transfer to the higher level shell to fill or half-fill it. The electron occupies the higher energy level shell in violation of the Aufbau principle because the atom is more stable that way.
Full or half-full subshells are very stable and have a lower energy level than they would otherwise have. For a few elements, the normal sequence of energy levels is changed because of full or half-full subshells. For higher atomic number elements, the differences in the energy levels becomes very small, and the change due to filling a subshell is more common than at lower atomic numbers. For example, ruthenium, rhodium, silver and platinum are all exceptions to the Aufbau principle because of filled or half-filled subshells.
In the lower atomic numbers, the difference in energy levels for the normal sequence of electron shells is larger and exceptions are not as common. In the first 30 elements, only copper, atomic number 24, and chrome, atomic number 29, are exceptions to the Aufbau principle.
Of copper's total of 24 electrons, they fill up the energy levels with two in 1s, two in 2s, six in 2p, two in 3s and six in 3p for a total of 18 in the lower levels. The remaining six electrons should go into the 4s and 3d subshells, with two in 4s and four in 3d. Instead, because the d subshell has room for 10 electrons, the 3d subshell takes five of the six available elctrons and leaves one for the 4s subshell. Now both the 4s and 3d subshells are half full, a stable configuration but an exception to the Aufbau principle.
Similarly, chromium has 29 electrons with 18 in the lower shells and 11 left over. By the Aufbau principle, two should go into 4s and nine into 3d. But 3d can hold 10 electrons so only one goes into 4s to make it half full and 10 go into 5d to fill it. The Aufbau principle works almost all the time, but exceptions occur when subshells are half-full or full.