i·so·tope 
(īsə-tōp′)
(īsə-tōp′)n.
One of two or more atoms having the same atomic number but different mass numbers.
[ISO- + Greek topos, place (so called because the isotopes of a chemical element occupy the same position in the periodic table of elements).]
i′so·topic (-tŏpĭk) adj.
i′so·topi·cal·ly adv.