Weighing Atoms: Atomic Mass Units
Ionizing & Non-Ionizing Radiation
Atoms are so small that it doesn't make sense to use the same units we use every day, like ounces or grams. To make it easier to work with atomic weights, early radiation scientists developed new units on a more appropriate scale.
They decided to use the mass of a well-known, and very common, element as the basis for measurements of atomic mass. The new scale equated one Atomic Mass Unit (AMU) to the mass of the most common carbon atom, which has six protons and six neutrons, divided by 12. So one AMU is about the same as one proton, and also about the same as one neutron. (Since electrons are so much smaller that they contribute very little to the mass of an atom.) One AMU is less than 1.66x10-24 gram, which is 0.00000000000000000000000166 gram.