$(i)$ Isotopes: These are atoms of the same element having the same atomic number, but different mass numbers. For example, chlorine has two isotopes with atomic number $17$ but mass numbers $35$ and $37$ represented by

$_{17}^{35}\,Cl{\kern 1pt} $,  $_{17}^{37}\,Cl{\kern 1pt} $

$(ii)$ Isobars: These are atoms having the same mass number, but different atomic numbers i.e., isobars are atoms of different elements having the same mass number. For example, Ne has atomic number $10$ and sodium has atomic number $11$ but both of them have mass numbers as $ 22 $ represented by –

$_{10}^{22}Ne$ , $_{11}^{22}Ne$

Two uses of isotopes :

$\to $ One isotope of uranium is used as a fuel in nuclear reactors.

$\to $ One isotope of cobalt is used in the treatment of cancer.