Isotopes have different physical properties due to

Isotopes have same atomic numbers but different mass numbers. It means number of neutrons should differ for the isotope elements.

Ex: ${ }_{17}^{35}\text{C}l,{}_{17}^{37}\text{C}l$

${\text{For\hspace{0.17em}\hspace{0.17em}}}_{17}^{35}\text{C}l,\text{\hspace{0.17em}\hspace{0.17em}No.\hspace{0.17em}of\hspace{0.17em}protons\hspace{0.17em}\hspace{0.17em}p}=17;\text{\hspace{0.17em}\hspace{0.17em}No.\hspace{0.17em}of\hspace{0.17em}netrons\hspace{0.17em}\hspace{0.17em}n}=18$

${\text{For\hspace{0.17em}\hspace{0.17em}}}_{17}^{37}\text{C}l,\text{\hspace{0.17em}\hspace{0.17em}No.\hspace{0.17em}of\hspace{0.17em}protons\hspace{0.17em}\hspace{0.17em}p}=17;\text{\hspace{0.17em}\hspace{0.17em}No.\hspace{0.17em}of\hspace{0.17em}netrons\hspace{0.17em}\hspace{0.17em}n}=20$