An atom contains subatomic particles called protons, neutrons, and electrons.
The basic structure of an atom includes a tiny, relatively massive nucleus, containing at least one proton and usually one or more neutrons.
Outside of the nucleus are energy levels (also called shells), which contain one or more electrons. The energy levels are often called rings (see more discussion of the Bohr model below).
The neutrons have the greatest mass and have no charge. The protons have slightly less mass than the neutrons and are positively charged. The electrons have almost no mass and are negatively charged. The electrons move around the nucleus in energy levels.