 The physical explanation turned out to be electron spin. Electrons have an intrinsic property that is best observed with a modern version of the Stern-Gerlach experiment that used silver atoms. Here we use magnets and electrons directly. The device has a north and south pole shaped to create a magnetic field that is stronger near the tip. This varies the force on charged particles passing through. A magnet is sent through with the north pole up and the south pole down. The magnetic field creates a force that deflects the magnet upward as it passes through the field. As we change the orientation of the magnets being sent through, we see the change in the amount and direction of the deflections. The deflections depend on the orientation. This is as expected. When electrons are sent through the field, they too are deflected, but they always arrive at the screen deflected either up or down, never in between like the magnets. Each electron behaves as a magnet, but with only one of two possible orientations, up or down. This intrinsic property of an electron is called spin. It is interesting to note that whenever an electron in an atom changes state, the atom's angular momentum changes. For example, here an electron moves from a higher energy orbital with angular momentum to a lower orbital with no angular momentum. We see that the emitted photon carries away both the energy and the angular momentum, giving it a spin equal to one. With the poly exclusion principle and spin as the fourth quantum number, the full set of spectral lines, orbitals and geometries and interactions with each other fell into place. In fact, when we add this fourth quantum number to Schrodinger's equations, we can generate the entire periodic table of the elements.