Why are relative atomic and molecular masses averages?

Relative atomic and molecular masses are averages because they account for the different isotopes of an element and their abundances.

In more detail, atoms of the same element can have different numbers of neutrons, resulting in different isotopes. Each isotope of an element has a different mass number, but the chemical properties remain largely the same. The relative atomic mass of an element is not simply the mass number of one particular isotope. Instead, it is a weighted average that takes into account the mass numbers of all the isotopes of that element, as well as their relative abundances.

For example, chlorine has two main isotopes, chlorine-35 and chlorine-37. In a sample of chlorine, about 75% of the atoms are chlorine-35 and about 25% are chlorine-37. Therefore, the relative atomic mass of chlorine is closer to 35 than it is to 37, reflecting the greater abundance of chlorine-35.

Similarly, the relative molecular mass of a molecule is the sum of the relative atomic masses of the atoms in the molecule. This also takes into account the different isotopes and their abundances. For instance, the relative molecular mass of water (H2O) is calculated by adding together the relative atomic masses of two hydrogen atoms and one oxygen atom.

In conclusion, relative atomic and molecular masses are averages because they reflect the fact that elements naturally occur as a mixture of isotopes. This concept is fundamental to understanding the behaviour of atoms and molecules in chemical reactions.