An isotope of a chemical element is an atom that has a different number of neutrons (that is, a greater or lesser atomic mass) than the standard for that element.
The atomic number is the number of protons in an atom's nucleus. Atomic mass adds to that the number of neutrons in the nucleus. Each element has a typical atomic mass, but when the number of protons stays the same and the number of neutrons changes, you have an isotope. These can be stable, like Deuterium, an isotope of Hydrogen that has one extra neutron, or they can be radioactive, like Plutonium-239, which is a component of nuclear waste.
