<P> The negatively charged electron has a mass equal to ⁄ of that of a hydrogen atom . The remainder of the hydrogen atom's mass comes from the positively charged proton . The atomic number of an element is the number of protons in its nucleus . Neutrons are neutral particles having a mass slightly greater than that of the proton . Different isotopes of the same element contain the same number of protons but differing numbers of neutrons . The mass number of an isotope is the total number of nucleons (neutrons and protons collectively). </P> <P> Chemistry concerns itself with how electron sharing binds atoms into structures such as crystals and molecules . Nuclear physics deals with how protons and neutrons arrange themselves in nuclei . The study of subatomic particles, atoms and molecules, and their structure and interactions, requires quantum mechanics . Analyzing processes that change the numbers and types of particles requires quantum field theory . The study of subatomic particles per se is called particle physics . The term high - energy physics is nearly synonymous to "particle physics" since creation of particles requires high energies: it occurs only as a result of cosmic rays, or in particle accelerators . Particle phenomenology systematizes the knowledge about subatomic particles obtained from these experiments . Total known Isotopes of Hydrogen - Deuterium and tritium - 2 </P> <P> The term "subatomic particle" is largely a retronym of 1960s made to distinguish a big number of baryons and mesons (that comprise hadrons) from particles that are now thought to be truly elementary . Before that hadrons were usually classified as "elementary" because their composition was unknown . </P> <P> A list of important discoveries follows: </P>

Is there anything smaller than a subatomic particle