Name the atoms of a substance having same atomic number, but different mass numbers. Give one example of such a substance. How do the structures of such atoms differ?

The atoms of a substance having same atomic number, but different mass number are known as isotopes.

Example — Hydrogen has three isotopes namely protium ${}_1^1\text{H}${1}^{1}\text{H}, deuterium ${1}^{2}\text{H}$ and tritium ${}_1^3\text{H}${1}^{3}\text{H}. Each isotope in it's nucleus has one proton (Z = 1) but protium (${1}^{1}\text{H}$) has no neutron, deuterium (${}_1^2\text{H}${1}^{2}\text{H}) has one neutron and tritium (${1}^{3}\text{H}$) has two neutrons. There is one electron outside the nucleus in the atom of each isotope. Thus, the structures of these isotopes differ by the number of neutrons in their nuclei.