Q.Define work function of a metal. Mention its unit.

Work function is the minimum energy that must be supplied to an electron at the Fermi level of a metal to just free it from the surface, measured in electron-volts (or joules).

1. Inside a metal, free (conduction) electrons are held back from escaping the surface by the attractive forces of the positive ions in the lattice; a certain minimum amount of energy must be supplied to overcome this and free an electron.
2. The work function ($\phi_0$ or $W_0$) of a metal is defined as the minimum energy required to just liberate an electron from the metal surface (from the highest occupied energy level, the Fermi level) into the vacuum outside, with no kinetic energy left over.
3. It is a characteristic property of the metal, and varies from one metal to another (e.g. caesium has a low work function; platinum has a comparatively high one). …