Both electron volt and joule are the units of energy. Since energies at the atomic level are very small and are expressed in electron volt rather than Joule because Joule is a bigger unit. Hence the unit electron volt for energy is more convenient than joule when we are dealing with atoms and nuclei to express the energy.
What is an electron volt?
An electron volt is an energy that is obtained by an atom when it travels through a potential difference of one volt. The numerical value of 1eV is equaled to 1.6×10–19 joules. The electron volt is the unit of potential difference.