By Drew Lichtenstein
Updated Mar 24, 2022

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Isotopes are atoms of the same chemical element that differ in neutron count, leading to distinct atomic masses. While most isotopes are stable, some are radioactive and emit radiation as they decay. Neutrons, which carry no charge, reside in the nucleus alongside protons and contribute to an atom’s mass and structural stability. On the periodic table, the atomic mass number equals the sum of protons and neutrons.

Step 1

Determine the neutron count of the atom in question. In practice, this value is supplied in experimental data or databases, as inspecting individual atoms directly is both impractical and expensive.

Step 2

Refer to the periodic table and note the standard atomic mass of the element. Reliable sources such as the NIST Atomic Weights database provide precise values.

Step 3

Subtract the number of protons (the atomic number) from the standard atomic mass to obtain the typical neutron count for that element. If the neutron count you measured differs from this value, the atom is an isotope.

For detailed examples and advanced applications, consult IUPAC guidelines or peer‑reviewed literature in nuclear chemistry.