Webster's English Dictionary

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isotope

iso.tope \'i--s*-.to-p also 'i--z*-\ \.i--s*-'ta:p-ik, -'to--pik also 
   .i--z*-\ \-'ta:p-i-k(*-)le-, -'to--pi-\ \'i--s*-.to--pe-, -z*-; 
   i--'sa:t-*-pe-, -'za:t-\ n [is- + Gk topos place - more at TOPIC] 1: any of 
   two or more species of atoms of a chemical element with the same  atomic 
   number and position in the periodic table and nearly identical chemical 
   behavior but with differing atomic mass or mass number and different 
   behavior in the mass spectrograph, in radioactive transformations, and in 
   physical properties 2: NUCLIDE  - iso.to.pic aj