Definition of isotope noun from the Oxford Advanced American Dictionary

 

isotope

 noun
noun
NAmE//ˈaɪsəˌtoʊp//
 
(physics)(chemistry)
 
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one of two or more forms of a chemical element that have the same number of protons but a different number of neutrons in their atoms. They have different physical properties (= characteristics) but the same chemical ones radioactive isotopes the many isotopes of carbon
See the Oxford Advanced Learner's Dictionary entry: isotope