Isotopes are versions of the same element that have different numbers of neutrons. If you look on the periodic table of elements you will notice that underneath each element there is a number. This number is called the atomic mass. The atomic mass is the average mass of all isotopes of an element. For example, Carbon has several isotopes such as carbon 12, carbon 13 or carbon 14. Because we know each carbon atom has the same number of protons (6 protons) we can find the number of neutrons in each isotope by subtracting 6 from each. Carbon 12 would then have 6 neutrons (12-6=6), carbon 13 would have 7 neutrons (13-6=7) and carbon 14 would have 8 neutrons (14-6=8).