Isotopes : The atoms which have atomic number $Z$ is same but atomic mass number $(A)$ is different then such type of atoms are called the isotopes of each other.

Isotopes of hydrogen are ${ }_{1} \mathrm{H}^{1},{ }_{1} \mathrm{H}^{2},{ }_{1} \mathrm{H}^{3}$ in ${ }_{1} \mathrm{H}^{1}$ there is one proton but having none neutron. In ${ }_{1} \mathrm{H}^{2}$ there is one proton and one neutron and in ${ }_{1} \mathrm{H}^{3}$ there is one proton and two neutrons. Gold has 32 isotopes whose mass numbers range from $\mathrm{A}=173$ to $\mathrm{A}=204$.

As the atoms of isotopes have identical electronic structure they have identical chemical behaviour and are placed in the same location in the periodic table.

In addition ${ }_{6}^{12} \mathrm{C},{ }_{6}^{13} \mathrm{C},{ }_{6}^{14} \mathrm{C}$ are the isobaric nuclide of carbon and ${ }_{92}^{233} \mathrm{U},{ }_{92}^{235} \mathrm{U},{ }_{92}^{238} \mathrm{U}$ are the isobaric nuclide of uranium and ${ }_{3}^{3} \mathrm{Li}$ and ${ }_{3}^{4} \mathrm{Li}$ are the isotopes of lithium.