What is the basic difference between the terms electron gain enthalpy and electronegativity?
The basic difference is that electron gain enthalpy is the enthalpy change when an electron is added to an atom, making it a measurable quantity. In contrast, electronegativity is the ability of an atom to attract electrons.
Furthermore, electronegativity is not a measurable quantity and it varies depending on the element to which it is bound. It provides a means of predicting the nature of force that holds a pair of atoms together. Electron gain enthalpy, however, is an intrinsic property of the isolated atom.
Explanation
The textbook context distinguishes the two by defining electronegativity as the 'ability to attract electrons' and explicitly stating that it is 'not a measurable quantity' and 'varies depending on the element to which it is bound'. Electron gain enthalpy, conversely, is discussed in terms of the energy change ('enthalpy') when an electron is added to an atom, implying it is a measurable, intrinsic property of the isolated atom.