One of the consequences of the Pauli Exclusion Principle is that if two electrons are paired up in an atom or molecule and have the same energy (remember from chapter 3 that the chemical bonds that hold molecules together are made up of electrons that are shared between atoms), then they have to have opposite spin. We can then think of their spins as canceling out, and we refer to them as being in a spin singlet state, since they can only inhabit a single state. This is the normal state of pairs of electrons in atoms and most molecules.
