Electron Spin

To explain what the difference between the two electrons might be, two graduate students, Samuel Goudsmit and George Uhlenbeck, proposed that the electron was like a spinning top with its own internal angular momentum.
This became known as spin angular momentum. The special feature of the electron’s spin is that it has two allowed projections, which we call spin up and spin down. In helium you could have two electrons in the same E₁ wave pattern if they had different spin projections, for then they would not be in identical states. Because the electron spin has only two allowed projections, we cannot add a third electron to the E₁ wave pattern. Lithium’s third electron must stop at one of the higher energy E₂ standing wave patterns. Its energy is much less negative and therefore this electron is much less tightly bound than the first two electrons that went down to the E₁ wave pattern.