How to Solve Chemistry Isotope Problems

Chemists often have to make calculations involving isotopes.
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There are two types of chemistry problems involving isotopes: finding the number of subatomic particles in an isotope and determining the average atomic mass of an element with isotopes. Isotopes are atoms of the same element with different numbers of neutrons. Having different numbers of neutrons changes the mass of the atom. Different isotopes of an element occur in nature in a set percent abundance. Due to the occurrence of isotopes, it is necessary to calculate a weighted average when finding an element's average atomic mass.

Finding the Numbers of Subatomic Particles in Isotopes

    Determine the number of protons and electrons in an isotope by finding the element's atomic number on the periodic table. The atomic number is the whole number on the periodic table that increases by whole numbers as you go left to right, top to bottom on the periodic table. The atomic number is equal to the number of protons. The atomic number is also equal to the number of electrons, since an atom is electrically neutral.

    Identify the isotope's mass number. The mass number of an isotope is often written after the element's name. For example in carbon-12 the "12" is the mass number of this isotope of carbon. The mass number may also be written as a superscript in front of the elements symbol such as ^235U. The mass number of an isotope represents the mass of the isotope's protons and neutrons.

    Calculate the number of neutrons in an isotope, by subtracting the atomic number from the mass number. For example, carbon-12 has six neutrons, since the atomic number of carbon is six. Twelve minus six equals six.

Finding the Average Atomic Mass of an Element with Isotopes

    Identify the mass of each naturally occurring isotope and the percent abundance of each isotope. This information may be found in a chemistry reference book such as the "Handbook of Chemistry and Physics," or online reference sources such as

    Multiply each isotope's mass by its percent abundance.

    Add each product of mass times percent abundance of each isotope. This sum represents the weighted average atomic mass of the element.

    Check you answer to see if it makes sense. The weighted average atomic mass should be somewhere between the mass of the smallest isotope and the mass of the largest isotope.

    Things You'll Need

    • Periodic table
    • Calculator
    • Chemistry reference book


    • If the mass of the isotope cannot be determined, but the number of protons and neutrons for the isotope is given, the mass of that isotope can be obtained by adding the protons and neutrons together, as they compromise the majority of the atom's mass.

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