Thus, if the total energy required to liberate an electron from a metal is 4 eV* and the energy
already possessed by the electron is 0.5 eV, then additional energy required (i.e., work function) is 4.0− 0.5 = 3.5 eV. The work function of pure metals varies roughly from 2 to 6 eV. It depends upon the nature of metal, its purity and the conditions of its surface. It may be noted that it is desirable that metal used for electron emission should have low work function so that a small amount of energy is required to cause emission of electrons.