Each proton has a positive charge. Outside the nucleus, the rest of the atom is mostly empty space. Orbiting at incredible speeds in this space are very small pieces of matter called electrons. Electrons have almost no mass and are negatively charged. They move so fast, some models picture the electrons as a cloud around the nucleus.
Some electrons are closer to the nucleus than others.
These different distances from the nucleus are called shells or energy levels . Each energy level can only hold a certain number of electrons before it is full. For example the closest or first energy level can only hold two electrons. The second energy level can hold 8 electrons and they have more energy than those in the first shell. Click here to get a rough idea of what it might be like to fly through the electrons surrounding an atom and see the nucleus.