Atomic Configuration Exceptions

[fergus_to5501]

Can someone clarify for me the exceptions surrounding the atomic configurations, please? I'm confused as to why, for example, copper would be [Ar]4s1 3d10, not [Ar]4s2 3d9.

Thanks

[admin]

Exceptions occur in 3d, 4d and 5d orbitals; however, for MCAT purposes, only 3d is normally seen.

A d subshell that is half-filled or full (ie 5 or 10 electrons) is more stable than the s subshell of the next shell. This is because it takes less energy to keep an electron in a half-filled d subshell than a filled s subshell. For example, copper (atomic number 29) has a configuration of [Ar]4s1 3d10, not [Ar]4s2 3d9 as you would expect.

Similarly, chromium (atomic number 24) has a configuration of [Ar]4s1 3d5, not [Ar]4s2 3d4 where [Ar], of course, represents the electronic configuration for argon.