Carbon 12 is the mass number of Carbon. The mass number is the number of protons plus neutrons in that particular element. So this means that Carbon 12 has a total of 12 protons and neutrons. But how do you find out how many protons there are? Well if you look at the periodic table you can see that the atomic number (which is the number of protons) is 6. This means that (12-6) there is 6 protons and 6 neutrons. Now lets look at Carbon 14. The atomic number is still the same but the mass number is different. This means that there is 6 protons and 8 neutrons (14-6=8 neutrons). So the only difference between the two is the number of neutrons and this is exactly what an Isotope is. Most elements have different Isotopes of that element. Another isotope of Carbon is Carbon 13. IF YOU HAVE ANY OTHER QUESTIONS... JOIN BROAD SCIENCE GROUP IN THE SCIENCE CATEGORY AND ASK MORE QUESTIONS.