An isotope is an atom that has the same number of protons as another element but has less (or more) neutrons. Having a different number of neutrons changes how other elements form bonds with the isotope.
an atom that differs in the number of neutons and therefore mass. For example, carbon-12 and carbon-14 are 2 isotopes of the element carbon. carbon-12 is the most abundant isotope, and carbon-14 (which has 2 extra neutrons per atom) is a radioactive isotope that can be measured for purposes of carbon dating.