In metals the electrons in the outer shells are loosely bound to the atoms, hence they are free to move easily the metal surface but I cannot leave the metal surface. Such electrons are called free electrons.
These free electrons can be emitted from the metals, if they have sufficient energy to overcome the attractive pull of metal surface. The phenomenon of emission of electrons from the surface of a metal is called electron emission.
Electron emission can be done in the following four ways.
(I) Thermionic Emission: The process of election of electron from the metal surface by heating is called thermionic emission.
(II) Field Emission: The process of election of electron from the metal surface by applying electric field is called field emission.
(III) Photoelectric Emission: The process of election of electron from the metal surface by falling light on it is called photoelectric emission.
(IV) Secondary Emission: The process of election of electron from the metal surface by falling fast moving particles on it is called secondary emission.
A certain minimum amount of energy is required to be given to an electron to pull it out from the surface of the metal.
The minimum energy required by an electron to escape from the metal surface is called work function of the metal.
It is denoted by Ø₀ and measured in electron volt(eV).
It is depends upon the nature of the metal that means it varies from one metal to another.
Na= 2.75 eV and Pt= 5.65 eV and
1 eV = 1.6×10⁻¹⁹ J