Exceptions to Electron Orbitals

[RaviAmin1H]

I wanted to know all the exceptions that we will be required to know for the test. I mean the exceptions to the orbital rule. Electrons fill orbitals in ascending order, but sometimes electrons will fill up a later orbital and leave an earlier orbital empty. These are exceptions and I wanted to know what elements we needed to know.

[Mika Sonnleitner 1A]

The two particular exceptions we need to know are Copper and Chromium. Copper would have an electron configuration of [Ar]3d9 4s2, but notice that the d subshell is one electron away from being completely full. In order for the atom to become more stable, an electron would be needed to complete the d subshell so it is full, so we take away one electron from the s orbital and move it to the d orbital, so that the electron configuration is [Ar]3d10 4s1.

Similarly, Chromium would have an electron configuration of [Ar]3d4 4s2, but since the d subshell is one electron away from being half full, one electron should be added to the d subshell to make it more stable. So, we take an electron from the s orbital and move it to the d orbital, resulting in an electron configuration of [Ar]3d5 4s1.

Although we technically only need to know these two elements for the exception, the exception applies to all elements in groups 6 and 11. Elements in groups 6 and 11 would have an electron configuration that includes nd4 or nd9, therefore it would be more stable if the d subshell was half full or completely full. In order to achieve this, one electron would have to be removed from the preceding s subshell. These exceptions would apply to all elements in the same groups as Chromium and Copper.

[Chem_Mod]

Also, remember why the half-filled orbitals are favorable for elements like Cr. The unpaired electrons do not experience any electron-electron repulsion in this scenario, the 4s electron that goes to the 3d has its own orbital rather than sharing.

[Kaelie Blanes-Ronda 2L]

Atoms would rather have half filled shells then a fully filled one a partially filled one, this increases stability in the atoms such as in the case of Copper and Chromium.