The number of protons, massive, positively charged #"nucular"# particles, gives #Z, "the atomic number,"# which 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=23,#the element is vanadium. The number of neutrons, massive, NEUTRALLY charged #"nucular"# particles, gives the particular isotope. Most elements have a number of accessible isotopes and their weighted average gives the atomic mass.
To keep hydrogen as an example, ALL hydrogen atoms have one nuclear proton; i.e. #Z=1#, this is what defines the nuclide as hydrogen. But some hydrogen nuclei have an extra neutron, to give the deuterium isotope, #""^2H#; and some fewer hydrogen nuclei have 2 extra neutrons, to give the tritium isotope, #""^3H#.
To conclude, if you are given the name of the element, you know its atomic number; and likewise, if you are given the atomic number, you know the name of the element. If you are given the number of neutrons then you know the isotope. It is worthwhile reviewing these facts, and getting them right the first time: it is fairly logical and straightforward.
