The atomic mass ma of an atom is the sum of the masses of all the subatomic particles in it. This is measured in atomic mass units u. The common isotope of carbon, carbon-12, has an atomic mass of 12 atomic mass units. Therefore for a standardized reference, the atomic mass is measured as the multiple of one-twelfth of carbon-12's atomic mass, which is 1.992646547×10−23gram.
Frequently Asked Questions
What's the difference between atomic mass and mass number?
The mass number A of an atom is the total number of protons and neutrons in the nucleus of the atom, whereas the atomic mass is the sum of their individual masses. When the mass number is expressed in atomic mass units, the value of the atomic mass of an atom will usually be close to that of its mass number.
What's the difference between atomic mass and atomic weight?
Atomic mass is measured for a particular isotope of an element, whereas the atomic weight is the average atomic mass of all the isotopes of an element observed on Earth. The better term for atomic weight is relative atomic mass. The atomic mass of an element is constant everywhere in the universe. But the relative atomic masses of elements vary from places where the isotope abundances differ.