Explain the emission of electrons from a metal. Define work function. Write its unit,and state the factors upon which the work function of a metal depends.

(N/A) Normally,electrons cannot escape from the surface of a metal because they are bound by the attractive forces of the positive ions in the metal lattice.
When the temperature of a metal is increased,the kinetic energy of the electrons due to thermal oscillation increases. When this energy exceeds the binding energy,the electron can escape from the metal surface,leaving the metal positively charged.
However,due to the electrostatic attractive force between the newly formed positive ion and the escaping electron,the electron is typically pulled back to the metal surface.
An electron can only permanently escape from the metal surface if its energy is greater than the potential energy barrier created by these attractive forces.
Work function: The minimum amount of energy required to remove an electron from the surface of a metal is called the work function of that metal.
Work function is denoted by $\phi_{0}$. Its $SI$ unit is $eV$ (electron volt).
Factors affecting work function: The work function of a metal depends on the nature of the metal and the condition of its surface (such as impurities or surface finish).