The difference between relative abundance and percentage abundance

The 'relative abundance' of an isotope means the percentage of that particular isotope that occurs in nature. Most elements are made up of a mixture of isotopes. The sum of the percentages of the specific isotopes must add up to 100%.

Percent abundance describes the prevalence of each of an element's isotopes in nature. The percent abundance of each isotope is used in the calculation of an element's average atomic mass.