How to Get Mass Number by Using Mass and Percent Abundance ?

The number of protons in an atomic nucleus is called the atomic number, and it is what distinguishes different elements, or types of atoms. All hydrogen atoms, for example, have nuclei with one proton. Isotopes are different forms of the same element which contain the same number of protons but different numbers of neutrons in the nucleus. Because protons and neutrons weigh the same, approximately one atomic mass unit, or AMU, the mass of any given isotope is equal to the number of neutrons plus the number of protons. The atomic mass numbers listed on the periodic table are a weighted average of all the naturally occurring isotopes of that element. Calculating this weighted average requires knowing the mass of each isotope and its percent abundance in nature.

Things You'll Need

• Calculator

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Instructions

1. • 1

     Multiply the mass of the lightest naturally occurring isotope by its percent abundance. The percentage should be written or entered into your calculator as a decimal. For example, 41 percent should be written as 0.41. Write down the result of the calculation and label it "isotope 1" so you don't lose track later on.

   • 2

     Multiply the mass of the second lightest isotope by its percent abundance, and label the result as "isotope 2." Perform this calculation again for the next heaviest isotope, and repeat until you have gone through all the naturally occurring isotopes.

   • 3

     Add together all the labeled results. The result is the mass number of the element in question. As long as your original data was correct, it should match the mass number on the periodic table.