What is the work function of a metal?

The correct answer is Option (2) → The minimum amount of energy required to extract an electron from the metal surface.

Option (1) The maximum amount of energy required to extract an electron from the metal surface. — Incorrect. There is no upper limit. An electron given more energy than needed still escapes. The extra energy appears as its kinetic energy.

Option (2) The minimum amount of energy required to extract an electron from the metal surface. — Correct. A free electron is held inside the metal by the attraction of the positive ions. The work function \( \phi_0 \) is the least energy it needs to overcome this pull and escape. It is usually measured in eV.

Option (3) Any amount of energy required to extract an electron from the metal surface. — Incorrect. The work function is one definite value for a given metal and surface, so it cannot be any amount. An electron given less energy than \( \phi_0 \) does not escape at all.

NCERT (2026-27):
Electron 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. This minimum energy required by an electron to escape from the metal surface is called the work function of the metal.”