Glad you asked. Pull up a chair and we'll tackle this one. We need to do a little review before we confront the isotope issue. Ready? Then let's have at it. An atom is an atom, but it becomes a particular element when we know the number of protons in its nucleus. Each element has a unique number of protons in its nucleus, and that is what determines what element it is. Hydrogen has one, helium has two, etc. But the kicker is that, though each element has a specific number of protons, it can have different number of neutrons in the nucleus of one of its atoms and still be that element. Same element (same number of protons), but different numbers of neutrons. Different atomic configurations of a given element are called isotopes of that element. Take helium for example. It has two protons (which is what makes it helium), but it can appear with one or with two neutrons. Each of these is an isotope of helium, and each one is stable, meaning it will not spontaneously undergo any atomic transformation. One other thing is that there are about a million atoms of He-4 for every atom of He-3. There are other isotopes of helium with three, four and more neutrons, but these are artifically made and are unstable. They will decay in a fairly short time. Now we've covered isotopes. The mass number (or atomic mass number or nucleon number) is simply the number of protons plus the number of neutrons in an atom. That's all. If we talk about, say, an atom of U-235, which is a fissionable isotope of uranium, the 235 is the atomic mass number. The element uranium has the atomic number 92, which means that there are 92 protons in its nucleus. If we subtract that 92 from the 235, we get 143 as a result, and that will be the number of neutrons in the nucleus of that isotope of 92U. Simple and easy. One more example. Carbon has an atomic number of 6, and carbon-14 has 14 minus 6 = 8 neutrons in it. Now you've got the scoop on isotopes, mass numbers and neutron counts.