Table of Contents
- What do you mean by free electrons?
- Work function of a metal
- Unit of work function
- Work Functions of some Photosensitive Metals
What do you mean by free electrons?
In metals, the electrons in the outer shells of the atoms are loosely bound. They move about freely throughout the lattice of positive ions. Such loosely bound electrons are called free electrons.
Work function of a metal
The free electrons remain confined to the conductor and cannot leave its surface at ordinary temperature and under moderate electric fields. The moment an electron comes out of a metal surface with its negative charge (- e), the metal surface acquires an equal positive charge (+ e) and pulls it back. There is thus a potential barrier at the metal surface which the free electrons have to overcome in order to just escape from the metal surface.
The minimum amount of energy required by an electron to just escape from the metal surface is called work function of the metal.
Factors on which work function of a metal depends
The work function depends on (i) the nature of the metal and (ii) the conditions of its surface. It is generally denoted by W0 (or ϕ0) and measured in electron volt (eV).
Unit of work function
Electron volt
One electron volt is the kinetic energy gained by an electron when it is accelerated through a potential difference of 1 volt.
Energy gained by electron
= Work done by electric field = qV
∴ 1 eV = 1.602 ×10-19C × 1 V
or 1 eV = 1.602 × 10-19 J
Electron volt is a commonly used unit of energy in atomic and nuclear physics.
Work Functions of some Photosensitive Metals
| Metal | Work function (eV) | Metal | Work function (eV) |
| Cs | 2.14 | Al | 4.28 |
| K | 2.30 | Hg | 4.49 |
| Na | 2.75 | Cu | 4.65 |
| Ca | 3.20 | Ag | 4.70 |
| Mo | 4.17 | Ni | 5.15 |
| Pb | 4.25 | Pt | 5.65 |
It may be noted from the above table that the work function of platinum is the highest (W0 = 5.65 eV) while it is lowest for caesium (W0 =2.14 eV).