[en] In order to understand nuclear reactions or transformations it is often very helpful to use models that explain scientific observations. There are a number of models that can be used to describe the nature of the atom; however, for our purposes, a very simple model can be used. An atom can be considered to be a dense core of particles called protons and neutrons forming a positively charged nucleus, surrounded by a cloud of orbiting electrons. Protons have a positive electric charge equal in magnitude to that of an electron. Neutrons have no charge and are approximately equal in mass to the protons. Collectively, the protons and neutrons are referred to as nucleons