Atoms of the same element can sometimes have different masses, so called isotopes of the element, and there must be other particles responsible for these different mass isotopes. It was also seen how bombardment with alpha particles could sometimes cause the ejection of some unknown and uncharged particles from within atoms. Finally, in 1932, James Chadwick, again using alpha particle scattering techniques, was able to consider in more detail the structure of nuclei and demonstrated the final atomic constituent at these scales - the neutron. These are similar in mass to the proton but are uncharged and there can be a variable number of them in atoms of the same element. Not all of these isotopes will be stable or occur naturally in nature and as a rule of thumb there are generally a similar number of, or slightly more, neutrons than protons.
Elements can have a different number of neutrons.
The chemical properties stay the same but the atom mass differs. In the above picture there is a hydrogen, deuterium, and tritium atom. It is currently believed that the electrons are truly fundamental particles but that the nucleons are themselves constructed from still smaller particles called quarks. There are two types of quark which between them make up the protons and neutrons. Both nucleons contain three quarks, protons contain two of one sort and one of the other whilst neutrons contain one of the first kind and two of the second.