an isotope occurs when the number of neutrons in an atom doesn't equal to amount of protons. A neutral atom has same number of protons as neutrons. So, if the number next to it isn't 2 times the atomic number, then that choice is an isotope. For example: If I had a Carbon atom, I know it has 6 protons, b/c its atomic number is 6. But, my atom only has 5 neutrons, not 6 neutrons. So it's an isotope. It will be labeled Carbon-11. The '11' comes from the sum of protons and neutrons.