Answer to Question #95545 in General Chemistry for Abigail Nguyen

Question

There are 21 elements that do not follow the Aufbau Principle. What are they?

Answer 1

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.