There are tiny particles called atoms. These particles are much smaller than the human eye can see. These tiny particles are composed of even smaller particles - protons, neutrons, and electrons. The total mass of the neutrons and protons in an atom is the mass number. It is possible for atoms of the same element to have a different number of neutrons and therefore, a different mass. They are called isotopes.
In order for an atom to be electrically neutral, there must be an equal number of protons and electrons. Electrons have a charge equal, but opposite to the proton. They are located outside of the nucleus in the electron cloud.