Let's take a simple molecule, H_2O. By definition, all H nuclei have ONE positively charged particle in their nuclei (i.e. Z=1), and all O nuclei have EIGHT positively charged particle in their nuclei (i.e. Z=8). Z, "the atomic number", defines the atom as a particular element: Z=1,H; Z=2,He; Z=8,O;....... Z=77,Ir; "etc."
The mass number represents the number of MASSIVE particles, positive and neutral, that are present in the element's nucleus. The neutral particles, so-called "neutrons", participate in VERY attractive interaction at almost impossibly short nuclear distances. And different numbers of neutrons give rise to the existence of "isotopes".
Most hydrogen nuclei have NO neutrons present in their nucleus; this gives the "protium isotope," ""^1H, which is approx. 99% abundant; a few hydrogen nuclei have ONE neutron present in their nuclei; this gives the "deuterium isotope," ""^2H, which is less than 1% abundant; a smaller proportion of hydrogen nuclei contain TWO neutrons; and this is the "tritium isotope," ""^3H.
As Z grows, most heavier elements have a number of stable isotopes. The "weighted average" of isotopic masses gives rise to the atomic masses printed on the Periodic Table. For a particular isotope, the mass number is simply the number of protons, Z, PLUS the number of neutrons present in that nucleus.
Anyway, you do have to have a grasp on these principles, but you don't have to remember atomic numbers, because a Periodic Table will be provided in every exam you sit in Chemistry and Physics. Good luck.
