Isotopes

Some elements have isotopes - atoms with different mass numbers, so different numbers of neutrons. For example, chlorine has a relative atomic mass of 35.45. This means that the average mass of all chlorine atoms is 35.45. From this we can infer that the most common isotope of chlorine has a mass of 35, but there must also be isotopes with a higher mass, such as 36 or 37.