A carbon atom contains 6 electrons and has a mass number of 12. The atomic number of carbon is 6, which means it has 6 protons. The mass number is the sum of protons and neutrons in the atom. To find the number of neutrons, subtract the number of protons from the mass number: Number of neutrons = Mass number - Number of protons = 12 - 6 = 6. Therefore, the carbon atom contains 6 neutrons.