And since it is an hydrogen isotope we can IMMEDIATELY say that #Z_"atomic number"=1#. Why? Because for elemental hydrogen #Z=1# BY SPECIFICATION. And since #Z# represents the number of massive, CHARGED nuclear particles, we say that ALL hydrogen nuclei contain one proton, i.e. one massive particle, with a formal POSITIVE charge.
And #99.98%# of all hydrogen nuclei in this universe is the #""^1H# isotope. But the nucleus could also contain massive particles of ZERO charge, and these are so-called neutrons... The deuterium isotope, #""^2H#, approx. #0.02%# abundant, contains the one (and defining proton), and ALSO a neutron, a massive particle of ZERO charge. And a trace percentage of hydrogen atoms contain TWO NEUTRONS, #"the tritium isotope"#, the which we would represent as #""^3H#. Note that we do not have to specify the atomic number #Z#, given that the atomic number, here #H#, ALREADY specifies that #Z=1#.
The mass number is simply the number of #"MASSIVE NUCULAR PARTICLES"#, for the #"protium, deuterium, and tritium isotopes"#, the mass numbers are #1,2,3# respectively... With me?
