Metals have free electrons. These electrons are responsible for their conductivity. But these free electrons cannot escape out of the metal surface. There is a reason why. If an electron come out of metal surface, the metal surface acquires a positive charge and pulls the elctron back to the metal. Hence the electrons are held inside the metal due to the attractive force of ions. Hence the metals can come out of the metal surface when we provide enough energy to the electron.
Different methods of electron emission
Thermionic emission: When we provide enough heat energy to the metal, the free electrons get extra energy. the electrons can come out of the metal using this extra energy.
Field emission: By applying a strong electric field ( of the order of 108Vm−1) to a metal, electrons can be pulled out of the metal.
Photo-electric emission: Light of different frequencies have different energies. More the frequency more will be the energy of the light. Hence when a light with suitable energy is incident on a metal surface, the electrons are emitted from the metal surface. These electrons are called photo-electrons.
We know that electrons may emit from a metal surface when light is incident on the metal. But a minimum energy is required for this process. This minimum energy is different for different metals. This minimum energy is called work function of the metal.
Unit of work function
Work function is the minimum energy required for a metal to emit electrons. Hence it has unit of energy. It may be Joules, or eV (electron volts). Generally the work function of the metals are of the order of electron volts. Hence they are generally represented in eVs.
Work function of common metals
The minimum amount of energy required to emit electrons from a metal surface is called the work function. It is characteristics of a metal. Work function for some metals (in evs) are as follows:
From above table it's evident that the alkali metals have low work function.