# Why carbon-12 is an isotope of carbon?

All carbon nuclei have the same atomic number, $Z$, the number of protons in the nucleus. For $C , Z = 6$, i.e. there are 6 protons in the carbon nucleus. If there were not 6 protons in the nucleus, the element would not be $C$. Most carbon nuclei have 6 neutrons; however, there are a very small percentage that have 7 or 8 neutrons, or even more neutrons. Approx. 99% of all carbon nuclei are carbon-12. It is the number of protons that determine the atomic number, that is the element's identity.