C-12 and C-13 are isotopes of the element carbon. Since they are both of the same element, they have the same number of protons. The number that follows the C indicates the number of neutrons. Isotopes are variations of the element with different number of neutrons. C-12 shows that that variation of carbon has 6 protons and 6 neutrons for a total mass of 12. C-13 shows that it has 6 protons but 7 neutrons for a total mass of 13
