ABSTRACT
The defining feature of each element is the number of protons in the atomic nucleus (see 4.1). In addition, each element has a unique relative atomic mass or atomic weight. Since 1961 it has been agreed that this should be the ratio of the average mass per atom of an element to one-twelfth of the mass of an atom of carbon-12 (the predominant isotope of carbon). For this purpose, carbon-12 has been assigned a value of 12. Note that because relative atomic masses are based on ratios, they have no units. The atomic masses of the elements are shown in Tables 3.1 and 3.2, and you will see that for most elements they are not whole numbers.
