An atom is the smallest particle which makes up a chemical element (e.g. hydrogen, oxygen, magnesium). All atoms are very small. Atoms are made of three components or particles: Protons, neutrons and electrons. The protons and neutrons are clumped together in a ball called a nucleus, at the center of every atom. The electrons orbit around the nucleus in its 'electron cloud'. Elements which have large nuclei, such as uranium and plutonium, can be made to fission.