How does ionisation enthalpy differ from electron gain enthalpy?

Ionisation enthalpy refers to the energy required to remove an electron from an atom, while electron gain enthalpy is the energy released when an atom gains an electron.

Ionisation enthalpy and electron gain enthalpy are two fundamental concepts in chemistry that describe the behaviour of atoms during chemical reactions. They are both measures of energy, but they describe different processes and have different implications for the properties of atoms.

Ionisation enthalpy, also known as ionisation energy, is a measure of the energy required to remove an electron from a gaseous atom or ion. It is always a positive value, as energy must be supplied to overcome the electrostatic forces of attraction between the electron and the nucleus. The higher the ionisation enthalpy, the more difficult it is to remove an electron from an atom. This is an important factor in determining the reactivity of an element, as elements with high ionisation enthalpies are generally less reactive.

On the other hand, electron gain enthalpy is a measure of the energy change when a gaseous atom gains an electron to form a negative ion. This process can either release energy (exothermic, negative electron gain enthalpy) or absorb energy (endothermic, positive electron gain enthalpy). The magnitude of the electron gain enthalpy can give an indication of an element's affinity for electrons, with elements that release a large amount of energy having a high electron affinity.

In summary, while both ionisation enthalpy and electron gain enthalpy are measures of energy associated with electrons, they describe different processes. Ionisation enthalpy is concerned with the removal of electrons and is always a positive value, while electron gain enthalpy deals with the addition of electrons and can be either positive or negative. Understanding these concepts is crucial for predicting the behaviour of atoms in chemical reactions.