It is made of even smaller particles called neutrons, protons and electrons arranged in a particular way. Neutrons and protons are clustered together in the center of the atom, called the nucleus . 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.