The periodic table of elements offers a small amount of information about the various elements, such as the atomic number and the atomic weight--also known as the atomic mass. One thing that isn't located on the periodic table of elements is the mass number, which is the number of particles that are located within an atom's nucleus. Fortunately, you can calculate this mass number with the information that is already on the table of elements.
- Periodic table of elements
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Instructions
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1
Locate the element on the periodic table of elements (see Resources section).
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2
Look for the atomic weight, which is the number below the element's abbreviation. For example, Li stands for lithium, and if you look below "Li" on the periodic table of elements, you see that it has an atomic weight of 6.941.
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3
Round the atomic weight to the nearest whole number to get the mass number of that element. For instance, the mass number of lithium is 7.