Isotopes were first discovered by Soddy in radioactive elements while isotopes of nonradioactive elements were discovered by Richard.
Isotopes means same position ( iso :same : topes : position )
“Atoms of same element having same atomic number but mass number is different, are called isotopes.”
Ex. (i) 17Cl35, 17Cl37
(ii) 8O16, 8O17, 8O18
(iii) 7N13, 7N14, 7N15
(iv) 1H1, 1H2, 1H3