(i) Each isotope has the same number of "nuclear protons", i.e. where the proton is a massive, fundamental nuclear particle with a unit positive charge. The number of protons gives Z, the atomic number, whose number determines the identity of the element: Z=1, the element is "hydrogen", Z=2, the element is "helium"; Z=3, the element is "lithium";........Z=25, the element is "manganese".
(ii) If the species are neutral, the isotope has the same number of "electrons" as nuclear protons; i.e. Z", the atomic number"-="number of electrons".
(iii) And if it is the same isotope, each nucleus contains the same number of "neutrons", where the neutron is a massive, fundamental particle of zero charge. The number of protons and neutrons gives the identity of the isotope.
There are three common isotopes of hydrogen: "protium," ""^1H;"deuterium," ""^2H, and "tritium," ""^3H. Given that you know Z for "hydrogen", how many neutrons does each isotope contain?
All of this is fairly straightforward (well I think so!). If you can digest this, you will get easy marks on an A-level examination. You don't have to remember Z, but you do have to remember what Z and "mass number" mean.
