For example , all atoms with 6 protons in their nuclei are of the chemical element carbon , and all atoms with 92 protons in their nuclei are of the element uranium . For example , all carbon atoms contain 6 protons in their nucleus ; so the atomic number `` Z '' of carbon is 6 . Carbon atoms may have different numbers of neutrons ; atoms of the same element having different numbers of neutrons are known as isotopes of the element .