That conversion of mass to energy is the basis for nuclear energy, which is discussed elsewhere. Thus, when you see the value of the atomic weight for an element, it will usually be a number slightly less than the sum of the particles. Simply round the number off to determine how many particles are normally involved. A good model that helps explain chemical properties is the Solar System Model of the atom. It states that electrons rotate in orbits around the atom's nucleus, just like the planets rotate around the sun in our solar system.
Electrons rotate around nucleus, just like the planets around the Sun.
Note that this picture of the atom is just a model or a simple picture of what the atom is like. Newer theories describe the electrons not as distinct particles rotating around the nucleus, but instead they are pictured as fuzzy clouds of material. If you go into more advanced Chemistry, you will see newer descriptions of the atom and its electrons, such as the electron cloud model. For our purposes though, the more simple solar system model of the atom is much easier to visualize and still explains many of the things that go on in Chemistry. Although the electrons rotate around the nucleus, similar to the planets around the sun, there are usually more than one electron in an orbit, as opposed to our Solar System.