[Chemistry Class Notes] on Electron Gain Enthalpy of Elements Pdf for Exam

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 energy released in the chemical reaction, the more the electron gains 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. 

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 shells. 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. 

Measurement and Use of Electron Affinity

Because their energy levels may be modified by contact with other atoms or molecules in a solid or liquid form, this feature is solely utilized to detect atoms and molecules in the gaseous state. Robert S. Mulliken developed an electronegativity scale for atoms based on a list of electron affinities, equivalent to the average of electron affinity and ionization potential. Electronic chemical potential and chemical hardness are two further theoretical ideas that involve electron affinity. Another illustration is that a molecule or atom with a higher positive value of electron affinity than another is referred to as an electron acceptor. Whileone with a lower positive value is referred to as an electron donor. Charge-transfer reactions might happen when they are brought together.

One-Electron Reduction

In organic chemistry, a one-electron reduction is the movement of a single electron from a donor chemical to an organic substrate. It distinguishes between two-electron organic reductions like hydride transfer processes. A radical anion is frequently the first step in a one-electron reduction, which then participates in secondary reactions. Proton removal from alcohol is the secondary reaction in the Birch reduction. A dissolving metal reduction is another name for this reaction. In the liquid ammonia or sodium system, alkyne reduction to an alkene continues the similar pattern. A proton from ammonia is abstracted by the first radical anion intermediary and transferred to the free radical. The anion is formed via a second one-electron transfer, which additionally extracts a proton from the neutral alkene.