What is electron gain enthalpy?
Electron gain enthalpy is often confused with electron affinity but to make it simpler to understand we can say that it is the energy that is released when a neutral isolated gaseous atom accepts an extra electron and forms a gaseous negative ion, called an anion. Electron gain enthalpy is denoted by ΔegH. It is a measure of the strength with which an extra electron is bound to the element. The greater the amount of energy released in the reaction, the higher is the electron gain enthalpy of the element.
The process of accepting electrons in an atom can either be exothermic or endothermic, depending on the element’s nature. Electron gain enthalpy is measured in electron volts per atom or kJ per mole.
Generally, there is a release of energy when an electron is added to an atom, and the electron gain enthalpy for such elements is negative.
The electron gain enthalpy of halogens is quite negative as these atoms only need one electron more to achieve the nearest noble gas configuration. In contrast, noble gases have a highly positive electron gain enthalpy. The extra electron must be placed in the next higher principal quantum level, which requires lots of energy.
Factors affecting electron gain enthalpy
Electronic configuration: Elements that have exactly half-filled or completely filled orbitals are very stable. Hence, more energy has to be supplied to add an electron to these elements. Therefore, their electron gain enthalpy has large positive values.
Atomic size: As the atom’s size increases, the distance between the nucleus and the last shell that receives the electron increases. This, in-turn decreases the force of attraction between the nucleus and the accepted electron. Hence, the electron gain enthalpy for such atoms is relatively less negative.
Nuclear charge: As we move forward in the periodic table, the nuclear charge of an element increases, the enthalpy becomes more negative. This is due to the fact that the force of attraction between the nucleus and the incoming electron increases as there is an increase in nuclear charge.
Variation of Electron Gain Enthalpy within a Group
The electron gain enthalpy becomes less negative as we move down a group. This happens because as we move down, both the atomic size and the nuclear charge of the element increase. But the effect of the increase in atomic size is much higher than the nuclear charge.
With the increase in atomic size, the attraction of the nucleus for the accepted electron decreases. Hence, the electron gain enthalpy becomes less negative.
Chlorine has the most negative electron gain enthalpy.
Variation of Electron Gain Enthalpy along a Period
The electron gain enthalpy of elements becomes more negative as we move from left to right in a period. As we move from left to right, the atomic size decreases, and the nuclear charge increases. Both of these factors increase the attraction by the nucleus for the incoming electron.
Halogens have the most negative electron gain enthalpy. As we move from Chlorine to iodine, the electron gain enthalpies become less negative due to the subsequent increase in their atomic size. The distance of the nucleus from the subshell which receives the additional electron increases; hence the force with which it is attracted by the nucleus decreases. Therefore the electron gain enthalpy becomes less negative as we move down the group from Cl —-> Br —-> I
Fluorine has Less Negative electron gain enthalpy than Chlorine.
This happens due to its (fluorine) small size. The electron-electron repulsion in the relatively compact to 2p subshell is comparatively large because of its smaller size. Hence the incoming electron is not accepted with the same ease as is the case with Chlorine.
Noble Gases have positive electron gain enthalpy
These elements have atoms with a completely filled subshell. Due to this, there is no place in their valence orbitals, and the additional electron has to be placed in an orbital of the next higher shell. As a result, energy has to be supplied for the addition of more electrons.