Since both are neutral, we know that each has 6 electrons as well. The sum of protons plus neutrons equals the mass number 12 in 12-C, so carbon-12 has 12 – 6 protons = 6 neutrons. The sum of protons plus neutrons equals the mass number 13 in 13-C, so carbon-13 has 13 - 6 protons = 7 neutrons.