The element an atom is determined by the number of protons in its nucleus. This number of protons is called the atomic number, which is unique for each element. For example, an atom with 1 proton is hydrogen, an atom with 6 protons is carbon, and so forth. Changing the number of protons in an atom changes the element itself, while changes in the number of neutrons or electrons do not change the elemental identity. The atomic number is the defining characteristic of an element.