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Key Idea 1
Concept Question: What is an isotope?

The basic structure of the atom is a nucleus surrounded by electro-magnetic fields in which moving electrons reside. Inside the nucleus reside nucleons: neutrons and protons. When an atom is characterized by a unique number of nucleons, we refer to it as a nuclide. Different numbers of neutrons and/or protons result in different nuclides.

If two atoms have different numbers of protons, they are different elements. However, if two atoms have the same number of protons, but different numbers of neutrons we refer to them as isotopes.

Two terms we use to identify nuclides (isotopes) are atomic number and mass number. Two atoms with the same atomic number, but different mass numbers (same number of protons, different number of neutrons), are called isotopes, or isotopic nuclides.

Having different numbers of neutrons changes the mass of these atoms, so isotopes have slight variations in their physical and chemical behavior. Some elements have many different isotopes, some only have a few, and some have no stable isotopes at all.

A particular isotope can be described in several ways. If we were discussing the isotopes of carbon and wanted to specify the isotope with a mass number (A) of 12 we would say "carbon twelve," and this could be written as carbon-12, or in a symbolic form with the mass number as a superscript: 12C. This symbolic form can also include the atomic number (Z) as a subscript, as in Carbon twelve in symbolic form. A capital C with the number 12 in superscript at the upper left and the number 6 in subscript at the lower left.

General symbolic form for isotopes,using carbon-12
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