Questions on exceptions to Aufbau principle.. Please help

[runner940]

Questions on exceptions to Aufbau principle. . . Please help!

Homework Statement

Explain how and why Co and Mo break the Aufbau principle in their respective e- configuration.

Homework Equations

The Attempt at a Solution

I was out of class when my teacher explained this, please help me. . .

 

[Borek]

Do you know what Aufbau principle is? Read about it in your book, you may also consult wikipedia. Then take a look at Co and Mo electron configuration - are these expected when applying Aufbau principle?

 

[Blueshift5]

The Aufbau principle states that electrons are added to the lowest energy orbital first before moving to higher energy orbitals. However, there are some exceptions to this rule, such as in the cases of Co (cobalt) and Mo (molybdenum).

In the case of Co, the electron configuration is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁷. This is because the 3d orbital has a slightly lower energy than the 4s orbital. Therefore, the electron from the 4s orbital moves to the 3d orbital, making the electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁸ 4s¹. This is known as the "half-filled subshell" exception, where having a half-filled d subshell is more stable than having a partially filled s subshell.

Similarly, in the case of Mo, the electron configuration is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d⁵. Here, the 4d orbital has a slightly lower energy than the 5s orbital, so the electron from the 5s orbital moves to the 4d orbital, making the electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 4d⁶ 5s¹. This is known as the "completely filled subshell" exception, where having a completely filled d subshell is more stable than having a partially filled s or p subshell.

These exceptions occur because of the underlying electronic structure and the energy levels of the orbitals. While the Aufbau principle is generally followed, there are exceptions based on the specific electronic configurations of certain elements. These exceptions are important to understand in order to accurately predict the electron configurations of elements. I hope this helps clarify the concept for you.