The right answer is 2.
The number of protons contained in a nucleus (called an atomic number) is characteristic of a chemical element. For a given atomic number, the number of neutrons defines different "types" of this element: isotopes. The variation of the number of protons of the nucleus of an atom, during a nuclear reaction for example, causes a change of the element studied.
I believe the answer to your question is none of the above
The elements are listed in order of increasing the atomic number. Its the number that is the number of protons in the nucleus of an atom.
