## Chem – College: Exceptions to the Electron Configurations

What sections should I know before attempting to learn this section?

Why can electrons configurations in the D orbitals change?

Some electron configurations tend to fill electrons in an odd way. These are the electron configurations that are exceptions to the rule. They mainly tend to focus on the s-orbitals and d-orbitals since these two orbitals are about the same distance from the nucleus (they have about the same energy). These exceptions to the rules start when you enter the 4s and 3d region of the orbitals displayed on the periodic table orbitals link here. The 4s orbitals are slightly closer to the nucleus (slightly lower energy) than the 3d so that is why the 4s orbitals are displayed first on the periodic table. However, they do not always fill first. It depends on the exact number of electrons or electron arrangement that a given element has. The exceptions come from one over arching concept. The d-orbitals prefer to have either 1 electron in each orbital or 2 electrons in each orbital. These two arrangements of electrons in the d-orbitals are more stable and therefore tend to happen when it is possible.

Examples: Give the abbreviated (noble gas) electron configurations for the elements below. Use the orbital periodic table.

 V [Ar] 3d5 Cr [Ar] 4s13d5 Ni [Ar] 3d10 Cu [Ar] 4s13d10

VIDEO Electron Configuration Exceptions Demonstrated Example 1: What is the abbreviated electron configuration for Mo? Use the orbital periodic table.

Step 1:

What is the nearest noble gas above Mo?

What should be written down so far: [Kr]

Step 2:

From Kr where do we continue the electron configuration?

Answer: into the 5s or 4d.

Step 3:

How many total electrons are in the 5s and 4d orbitals?

Step 4:

Where do I distribute the electrons between the 5s and 4d orbitals?

Answer: first give 5 electrons to the 4d orbitals. Then give whatever is left over to the 5s orbitals. In this case the 5s orbitals will have 1 electron.  