The identity of an element is defined by its number of for example, every atom of carbon has protons and only atoms with 6 protons are however, atoms of the same element can have number of neutrons. when atoms of the same element have different numbers of neutrons they are called . since the mass of an atom is determined by the total number of protons and neutrons, isotopes of the same element will have different despite having the numbers of protons. also, isotopes of the same element will have similar behavior. this is because chemical is based on the number of and isotopes only have different numbers of