Electron Spin
Additionally to the quantum numbers n, l and m, which label its orbital, spin is an electron is given an additional quantum number relating to an intrinsic property, which is related with an angular momentum about its own axis and a magnetic moment. The rotation of the planets about their axes is sometimes employed as an analogy, but this can be misleading as spin is a necessarily quantum phenomenon, which cannot be described by classical physics. The electron's direction of spin can take one of only two possible values that is represented by the quantum number ms, which can have the values 1/2 and -1/2. Frequently these two states are called spin-up and spin-down or denoted by the α and β.
Electrons in similar orbital with different ms values are said to be paired. Electrons with similar ms value have parallel spin. Atoms, solids and molecules and with unpaired electrons are attracted into a magnetic field, a property termed as paramagnetism. The paired electrons' magnetic effects cancel out and substances with no unpaired electrons are weakly diamagnetic, being repelled through magnetic fields.
Experimental proof for spin comes from an analysis of atomic line spectra, which depict that states with orbital angular momentum (l>0) are divide into two levels by a magnetic interaction termed as spin-orbit coupling. It takes place in hydrogen but is extremely small there; spin-orbit coupling increases with nuclear charge (Z) about as Z4 and so becomes more important in heavy atoms. Dirac's equation, that incorporates the influences of relativity into quantum theory, that provides a theoretical interpretation.