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The Pauli Exclusion Principle states that, in an atom or molecule, no two electrons can have the same four electronic quantum numbers. We are aware that in one orbital a maximum of two electrons can be found and the two electrons must have opposing spins. That means one would spin up ( +1/2) and the other would spin down (-1/2).
We have the first three quantum numbers \(n=1\), \(l=0\), \(m_l=0\). Only two electrons can correspond to these, which would be either \(m_s = -1/2\) or \(m_s = +1/2\). As we already know from our studies of quantum numbers and electron orbitals, we can conclude that these four quantum numbers refer to 1s subshell. If only one of the \(m_s\) values is given then we would have 1s1 (denoting Hydrogen) if both are given we would have 1s2 (denoting Helium). Visually this would be represented as:
As you can see, the 1s subshell can hold only two electrons and when filled the electrons have opposite spins.
This material is based upon work supported by the National Science Foundation under Grant Number 1246120