The main difference between carbon-12 and carbon-14 is the number of neutrons present in each compound. Carbon-12 has six neutrons, while carbon-14 has eight neutrons, explains About.com. The differing number of neutrons is responsible for numerous differences between the two compounds.
Carbon-12 is a stable isotope. Carbon-14 is not stable and undergoes radioactive decay. This is a natural process for unstable isotopes. The radioactive decay of this isotope is helpful in determining the age of objects that have been around for thousands of years. Carbon-14 is a useful tool to archaeologists because of its stable half life of 5720 years. Carbon-14 is heavier than carbon-12 because it has more neutrons, though both isotopes have the same atomic number because they have the same number of protons. Since carbon-12 and carbon-14 have a different number of neutrons, their atomic weights are not the same. A neutral atom of carbon-14 or carbon-13 would have six neutrons. Carbon-12 is the most abundant of the two isotopes and is found in the earth's crust, whereas carbon-14 is uncommon. Carbon-12 is the standard scientists use to measure atomic weights. Originally, oxygen was the standard for measuring the atomic weight of elements. Scientists Martin Kamen and Sam Ruben are responsible for the discovery of carbon-14 in 1940.