Until now, you have dealt with particles that do not change their identity. Changing the identity of a particle occurs when a particle decays to another particle (or, typically, set of particles), or when two or more particles fuse together. In these notes, you will read about a new unit of energy (the electron volt) and how to use energy to predict or explain particle decay.

For many situations, the unit of the Joule is quite useful. For very small particles like neutrons, protons, and electrons, a different unit is used typically. Consider the rest mass energy for the neutron,

$$E_{rest} = mc^2 = (1.6749\times10^{-27}kg)(3\times10^{8})^2 = 1.51\times10^{-10}J$$