# Spin quantum number of electron

Why is the spin quantum number of an electron +1\2 or -1\2 in an atom? Is this an experimental value or theoretical value?

• Actually, only the projection of the spin of an electron has a sign and is given in unints of $\hbar$. This fact was initially put forward by experiments but could later be explained using relativistic quantum mechanics. – Paul Nov 18 '16 at 9:44
• An electron has two quantum numbers the spin which is $1/2$ and the projection (also called magnetic or azimuthal) quantum number onto an axis which has values $\pm 1/2$. – porphyrin Nov 18 '16 at 12:44
• Here is the corresponding question (with answer!) on Physics.SE. – a-cyclohexane-molecule Sep 20 '18 at 0:10