The Pauli Exclusion Principle states that, in an atom or molecule, no two electrons can have the same four electronic quantum numbers . As an orbital can contain a maximum of only two electrons, the two electrons must have opposing spins.
This means if one is assigned an up-spin ( +1/2), the other must be down-spin (-1/2).
Electrons in the same orbital have the same first three quantum numbers,
e.g.,
, , for the 1 s subshell. Only two electrons can have these numbers, so that their spin moments must be either
or . If the 1 s orbital contains only one electron, we have one
value and the electron configuration is written as 1 s (corresponding to hydrogen). If it is fully occupied, we have two values, and the electron configuration is 1 s (corresponding to helium). Visually these two cases can be represented as
As you can see, the 1 s subshell can hold only two electrons and when filled the electrons have opposite spins.
Contributors
Sarah Faizi (University of California Davis)
Dr. Craig Fisher (Japan Fine Ceramics Center)
