Atomic Mass

The average mass of the atoms of an element in amu determined relative to the mass of the most abundant isotope of carbon (C-12) taken as 12 amu. eg the atomic mass of chlorine is 35.5 which is the average of the two isotopes of element chlorine ie Cl-35 and Cl-37 with atomic masses 35 and 37 amu respectively. (The actual mass of Cl-35 is 34.7 amu and that of Cl-37 is 36.6 amu). By averaging we get the atomic mass of chlorine element as follows.
(35×75+37×25)÷100=35.5
For pedants. This means that if we take an atom of chlorine from a lot, its atomic mass will be either 35 or 37 amu. So when we speak of atomic mass we speak average. In other words if we have a bag full of chlorine gass it is atoms of chlorine gass some weighing 35amu and some weighing 37amu. Similarly if we have a bag of HCl gas. Some of its molecules will have molecular mass corresponding to Cl-35 and some to Cl-37. If hydrogen is also considered, then there will be 5 types of HCl molecules on the bases of molecular mass for H-1 H-2 H-3 Cl-35 and Cl-37. The average combination is the molecular mass of HCl. Atomic mass is also called relative atomic mass or average atomic mass or atomic wieght.