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

The work function of a metal, denoted $\phi_0$ and conventionally measured in electron-volts (eV), is defined as the minimum energy an electron must be given to escape from that metal's surface -- in other words, the height of the surface potential barrier that a free electron must overcome to be emitted. One electron-volt is itself defined as the kinetic energy an electron gains when accelerated through a potential difference of exactly 1 V: since work done equals charge times potential difference, $1\ \text{eV}=qV=(1.602\times10^{-19}\ \text{C})(1\ \text{V})=1.602\times10^{-19}\ \text{J}$, giving a convenient, human-sized unit of energy for atomic and nuclear-scale problems where the SI joule would otherwise involve unwieldy powers of ten.

If a free electron already carries some kinetic energy inside the metal but this falls short of the full surface-barrier height, only the shortfall needs to be supplied externally to free it -- for example, if the barrier requires 3 eV and a particular free electron already has 0.5 eV, only $3-0.5=2.5$ eV of extra energy is needed, and this 2.5 eV is the work function as measured for that emission event. Work function is not a universal constant; it is a property of each individual metal and, more precisely, of the nature of that metal's surface, and its value ranges widely -- from about 2.14 eV for caesium up to about 5.65 eV for platinum, with intermediate metals like sodium (2.75 eV), copper (4.65 eV) and silver (4.70 eV) spread across the range. A metal with a smaller work function is more effective for electron emission of any kind, since less extra energy needs to be supplied to release its free electrons -- which is exactly why caesium and the other alkali metals, having the lowest work functions of common metals, are favoured as the photosensitive material in practical photoelectric devices such as photocells.