The difference is the number of neutrons.

Carbon 12, 13 and 14 are carbon isotopes, meaning that they have additional neutrons:

• Carbon 12 has exactly 6 protons and 6 neutrons ( hence the 12 )
• Carbon 13 has 6 protons and 7 neutrons
• Carbon 14 has 6 protons and 8 neutrons

Because molecular mass is calculated by adding the number of protons and neutrons, you could also say that the difference between these isotopes is mass ( if you have 1 mole of each isotope, carbon 14 would have the greatest mass ).

Carbon 14 is also radioactive with a half-life of 5700 years.