Atomic Masses

Individual atoms are too small to be weighed. Instead, the mass of one atom can be compared with that of another atom using an instrument called a mass spectrometer. The relative masses of atoms are called relative atomic masses. The measurement of relative atomic masses originally used hydrogen as the standard.

With the existence of isotopes, chemists now use a particular one as the standard--carbon-12, which gives a relative atomic mass of exactly 12. The relative atomic mass of an element is therefore the average mass of an atom of the element relative to one twelfth of the mass of an atom of the isotope carbon-12.

Relative atomic mass = average mass of atom of element
                                      1/12 * mass of atom of carbon-12

The symbol for relative atomic mass is Ar, where r stands for relative. Using the carbon-12 scale, the relative atomic mass of hydrogen is 1.0 and that of chlorine is…