What do you mean by successive electron gain enthalpies? Why is the second electron enthalpy of an atom positive?

Explanation: Electron gain enthalpy is the energy released or absorbed when an atom in the gaseous state gains an electron. The first electron gain is usually negative because the atom becomes stable after gaining an electron. For example, chlorine gains one electron to form Cl⁻.

Successive electron gain enthalpies occur when additional electrons are added. The second electron gain enthalpy is always positive because the negatively charged ion repels the incoming electron. To add another electron, energy must be supplied to overcome repulsion. Therefore, second enthalpy is positive. First is usually negative, but successive ones are positive due to electron-electron repulsion.