The free electrons in a metal if supplied with sufficient energy, from outside, leave the metal surface. The process of emission of electrons from the metal surface is called electron emission.
The minimum amount of energy required to emit electrons from a metal surface is called the work function (or threshold energy) of that metal.
It is generally expressed in electron-volt (eV). 1 eV = 1.6 x 10-19 J.
The emission of electrons from a metal surface when heat (or thermal) energy is imparted to it, is called the thermionic emission.
The electrons which are in orbits close to the nucleus are called the bound electrons.
They are tightly bound to the nucleus by the strong attractive force between the positive charge of the nucleus and negative charge of electrons.
When a large number of atoms come together to form a solid, the loosely bound electrons of the outermost orbit leave their individual atom and move freely inside the solid.These electrons are called conduction electrons.
Metals have a large number of free electrons, whereas non-metals have very few of them.
Factors affecting the rate of thermionic emission (NTS):
- The nature of the metal surface: Lower the work function of the metal, greater the rate of emission
- The temperature of the surface – Higher the temperature of the surface, greater the rate of emission
- The surface area of the metal – Larger the surface area of the metal, greater the rate of emission
Requisites for a good electron emitter:
- The work functionof the substance should be low.
- The melting pointof the substance should be quite high.
- Thoriated tungsten is used as an electron emitter.
- Tungsten coated with an oxide of either barium, cesium or strontium is preferred, to be used as an electron emitter.
Radioactivity refers to the particles which are emitted from nuclei as a result of nuclear instability. The most common forms of radiation emitted have been traditionally classified as alpha, beta, and gamma radiation.