The structure of the atom consists of a nucleus (protons and neutrons) and electrons circulating around it. The nucleus is quite dense so it has a large mass, but the electrons, which make up most of the overall mass of the atom, are much lighter. Electrons travel through the energy levels surrounding the nucleus, determining the chemical properties of the atom. Since electrons have a negative charge, they hold the atom together by attracting the positively charged protons in the nucleus. The chemical properties specific to an atom depend on the number of protons and neutrons in its nucleus. The number of protons (atomic number) of the atom is used to determine the chemical element of the atom.