[ 'ʌɪsətəʊp ]
■ noun Chemistry each of two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei.
Derivatives
isotopic adjective
isotopically adverb
isotopy noun ʌɪ'sɒtəpi
Origin
1913: from iso- + Gk topos 'place' (because the ~s occupy the same place in the periodic table).