Define the binding energy of an electron.

Hint : The binding energy of an electron is related to the electrostatic force of attraction which acts on it due to the nucleus. The potential energy due to this attractive force exists for the electron, from which the concept of binding energy can be explained

Complete step by step answer:
We know that an atom is composed of a nucleus, a cloud of the electrons. The electrons revolve around the nucleus under the effect of the electrostatic force provided by the nucleus. The electrons are negatively charged while the nucleus is a positively charged species, so there is an electrostatic attraction on the electrons by the nucleus. This electrostatic force of attraction provides the required centripetal force to the electrons, under the effect of which they revolve in the well defined shells. Since the positive charged nucleus and the negatively charged electrons are separated, so there is a potential energy of the electrons.
The binding energy for an electron is the energy required to be supplied to an electron revolving around an atom in a particular shell, to remove it completely from the effect of the nucleus. An electron is said to be removed when it is separated to an infinite distance from the nucleus. The potential energy of the electron at the infinity is taken as zero. So the binding energy of an electron is equal to the negative of its potential energy.

Note:
We should not get confused with the binding energy for a nucleon. Generally, the term binding energy is used for the nucleon particles, namely the protons and the neutrons. But we should remember that the concept of binding energy is defined for each pair of particles, which are bound together by a conservative force.