The same element with different neutron numbers
Proton number determines an element’s identity. Changing the neutron number produces a different isotope without changing the element. Carbon-12 and carbon-14, for example, both have six protons but contain six and eight neutrons respectively.
Mass number and atomic weight
The integer in an isotope’s name is its mass number, the total of protons and neutrons, not a sample’s average atomic weight. Natural samples often contain several isotopes, so tabulated atomic weights commonly reflect their masses and relative abundances.
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Stable and radioactive isotopes
Some isotopes are stable and others undergo radioactive decay. Carbon-14 decay is useful for dating under appropriate conditions, while stable isotopes have many applications of their own. The word isotope does not itself mean that a material is radioactive.
Tracing the movement of matter
Researchers measure isotope ratios or introduce identifiable isotopic labels to follow material through environmental and biological processes. Interpretation requires attention to sources, mixing and fractionation; a measured ratio by itself need not establish one unique origin.