What is an Isotope? An isotope is one particular form an atom of an element might take. Just as people may be thin, medium weight, or heavy, isotopes vary according to their weight. You will remember from the last chapter that the mass of an atom is determined by adding the number of protons to the number of neutrons in the nucleus. Since all the atoms of an element must have the same number of protons, the only thing that can vary to cause this change is the number of neutrons. This means that a "skinny" isotope would have a low number of neutrons. The "heavyweight" isotope would have the most neutrons. For example, there are three basic isotopes (forms) of carbon. The most common form has six protons, six electrons, and six neutrons. It is known as carbon-12, to designate the mass of the six protons plus the six neutrons. This is the skinny form. There is an extremely rare isotope of carbon known as carbon-13. This form has six protons, six electrons, and seven neutrons. It is the medium-weight form. Therefore, its atomic mass number would be 13. The least common, radioactive form of carbon is carbon-14. Carbon-14 contains six protons, six electrons, and eight neutrons per atom. This is the heavyweight form.

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