Electron Gain Enthalpy Trend
The electron gain enthalpy refers to the amount of energy that gets released upon the acceptance of an atom from any neutral isolated gaseous atom to form a negative gaseous ion (anion) in the process. It is often represented as ΔHeg and is measured in the unit of electron volts per atom or kJ per mole (kJ/mol). The electron gain enthalpy method represents the energy involved. It also tells about the strength of the extra electron that gets bound to the gaseous atom. The more amount of energy released in the chemical reaction, the more the electron gain enthalpy of the element. Such reactions can be both exothermic and endothermic in nature, meaning releasing and intaking of energy based on the constituent elements.
Electron Gain Enthalpy Example
Here’s an example of a similar process:
A(g) + e⁻ → A⁻(g)
Negative Electron Gain Enthalpy: It is represented by its negative values as the energy gets released, the halogen atoms gain stability by gaining electrons. As the halogens display a strong affinity to reach to the stable, noble gas state, the halogens have a higher negative electron gain enthalpy.
Positive Electron Gain Enthalpy: It is the process when the element shows a certain reluctance in accepting a new (generally the second atom). Since the noble gases have a high positive electron gain enthalpy, it places the extra gained electron into the higher maximum energy levels -leading to a highly reactive and unstable electronic configuration. As with the addition of one electron, the atoms then get negatively charged, and therefore the addition of the next electron often gets disrupted by electrostatic repulsion. Such reactions require a further supply of energy, causing the electron gain enthalpy of the second electron positive in nature.
The exciting feature of the positive electron gain enthalpy is that it gets more negative as it moves from left to right in a period as and when compared to coming from top to lower in a group. Generally, the variation of positive gain enthalpy is irregular in a group or a period.
As with the addition of one electron, the atoms then get negatively charged, and therefore the addition of the next electron often gets disrupted by electrostatic repulsion. Such reactions require a further supply of energy, causing the electron gain enthalpy of the second electron positive in nature.
For the following reaction with Oxygen, where it forms the O⁻ ion energy gets absorbed as the second electron faces electrostatic repulsion. Here's the entire mechanism:
O(g) + e⁻ → O⁻(g); (ΔHeg)₁ = -141 kJ
O⁻(g) + e⁻ → O²⁻(g); (ΔHeg)₂ = +780 kJ
Factors That Affect Electron Gain Enthalpy
Atomic Size: With the increase in atomic size, the overall distance between the nucleus and the last cell increases. This leads to the decrease in the force of attraction between the core and the newly added electron and therefore becomes less negative.
Nuclear Charge: With the increase in the total negative charge, the force of attraction with the newly added electron and the nucleus increases, leading to the enthalpy turning more negative in nature.
Electronic Configuration: The elements that have the exact half-filled or wholly filled orbitals, are generally very stable. For such elements, energy needs to be provided to undergo the addition of electrons. Therefore, the electron gain enthalpy of such elements is substantially large in value.
Electron Gain Enthalpy in Period and Group
The electron gain enthalpy of groups 16 and 17 varies to become less negative as you move down a group, because of the change in the atomic size, nuclear charge. An example of the same would be that of O where the electron gain enthalpy is -141 kJ/mol, while S has -200 kJ/mol, as the newly added electron goes into the smaller n=2 kernel. Since the small size of the element, the interelectronic repulsion increases, thus increasing the electron gain enthalpy in the process.
The electron gain enthalpy in the periodic table increases in its negativity moving from left to right in a period. As the atomic size decreases, the nuclear charge increases, thus increasing the chances of electron attraction. In the periodic table, the elements like Be, N, and Ne display positive electron gain enthalpy because of its half-filled degenerative shells. Here, Be and Ne have completely-filled shells while the N has half-filled p shell. Moving from Chlorine to Iodine in a periodic table, the electron gain enthalpies keep falling to lower negative values as an increase to their atomic radii.
Q1. Why is the Electron Gain Enthalpy of Oxygen and Fluorine Less Negative than the Corresponding Elements of the Third Period?
A. Since the elements of the second period inherently come in smaller atomic size than that of the third period, therefore the inter-electron repulsion occurs within the atom. Such an inter-atomic reaction creates a disruption in the acceptance of the additional electron, which requires more effort than others in the group. Therefore, Oxygen and Fluorine possess lesser negative electron gain enthalpy than the rest.
Q2. Why is the Electron Gain Enthalpy of Magnesium Positive?
A. Since the elements like Magnesium have the extra stability from the fully filled s-orbitals (3s2), the electron gain enthalpy is highly endothermic, since it would require an added energy to attract electrons. Therefore, the electron gain enthalpy would be positive in nature for elements that are perfectly half-filled or full-filled orbitals.