Identify the pairs which are not of isotopes.

a. \(_{6}^{12}X \ , \ _{6}^{13}Y\)
b. \(_{17}^{35}X \ , \ _{17}^{37}Y\)
c. \(_{6}^{14}X \ , \ _{7}^{14}Y\)
d. \(_{4}^{8}X \ , \ _{5}^{8}Y\)

1. a, d
2. b, c 
3. c, d 
4. b, d

(c, d)
(a) X612andY613 have same atomic number but different mass number.
(b) X1735andY1737 have same atomic number but different mass number. Both these pairs are isotopes to each other.
(c) X614andY714 have different atomic number but same mass number.
(d) X48andY58 have different atomic number but same mass number.
Both these pairs are isobars to each other.