Answer:
carbon-14 or carbon-6, sorry i'm not sure which one.
Explanation:
For example, carbon-12 has 6 protons and 6 neutrons and carbon-13 has 6 protons and 7 neutrons and both are stable isotopes, where as carbon-14 (a radioactive isotope) has 6 protons and 8 neutrons and will decay (Fig. 1).
