One of two or more atoms with the same atomic number that contain different numbers of neutrons (7)
I believe the answer is:
isotope
(Other definitions for isotope that I've seen before include "O, is poet involved in chemical element?" , "Variant element form" , "Form of atomic nucleus" , "One of the variants of an element sharing an atomic number but with differing chemical properties" , "Oops, tie up the atom" .)