Electrons and spin 1/2
Atomic and solid state physics can considered as principally as the physics of electrons. Of the massive particles that make up matter, neutrons, protons and electrons, the electron is the lightest and therefore the most affected by quantum mechanics.
One of the most amazing properties of electrons is that it carries an intrinsic angular momentum. This mean that is is "spinning" even when it is sitting by itself. This spin can not have any value but must be either +1/2 or -1/2 in units of h/(2p). The fact that the angular momentum can only have discreet values can be stated: "The spin (intrinsic angular momentum) of particles is quantized. We will further see that the half-integral vs. integral nature of the spin is of fundamental importance.
We will also see that the orbital states open to an electron in orbit around a nucleus are quantized. This means that only orbits with discreet energies are possible. The combination of the discreetness in the orbits and in the spin gives structure to atomic configurations.