#(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.
