Exploring the subject of elementary particles is rather like trying to find our way around an enormous zoo without the help of a guidebook to identify the different species of animal. How are we to make sense of it all? We will begin by summarizing some of the properties exhibited by the elementary particles. It is helpful to picture each particle as a tiny sphere that has three fundamental characteristics: electric charge, mass and spin. The different particles can be described in terms of these three basic quantities.

Electric charge is a familiar concept. Some particles carry it, but others do not. Those that do not are said to be electrically neutral. Likewise some particles have a mass, but others are massless. The mass of a particle contributes to its total energy, because mass is just another form of energy. Mass may be converted into energy and vice versa, and, indeed, a huge amount of energy may be produced from a relatively small mass.

The amount of spin that a particle carries determines its rate of rotation. We can view spinning particles as rotating about an axis. The electron is an example of a spinning particle. The spin of all elementary particles is severely restricted. Those particles that do not rotate have zero spin. Particles that do rotate have a spin that is directly related to that of the electron.

Elementary particles are divided into two main groups depending on the amount of spin that they carry.