$(a)$ Define the following terms with one example each:
$(i)$ Isotope
$(ii)$ Isobar
$(b)$ Name the elements whose isotopes are used in:
$(i)$ Nuclear Reactor
$(ii)$ Treatment of cancer

(N/A) $(i)$ Isotopes are atoms of the same element having the same atomic number but different mass numbers. Example: Hydrogen has three isotopes: Protium $(_{1}^{1}H)$,Deuterium $(_{1}^{2}H)$,and Tritium $(_{1}^{3}H)$.
$(ii)$ Isobars are atoms of different elements having the same mass number but different atomic numbers. Example: Argon $(_{18}^{40}Ar)$ and Calcium $(_{20}^{40}Ca)$ are isobars as both have a mass number of $40$.
$(b)$ $(i)$ The isotope of Uranium $(U-235)$ is used as a fuel in nuclear reactors.
$(ii)$ The isotope of Cobalt $(Co-60)$ is used in the treatment of cancer.