Electron Gain Enthalpy (EGH)

Electron gain enthalpy is the reason why some elements accept electrons while others don’t. It is the fundamental concept in chemistry that unveils how atoms interact with electrons. This property discusses the tendency of an atom to form negative irons as well as its chemical features across the periodic table. So, whether it is the explosive nature of halogens you want to understand or the behaviour of atoms, then electron gain enthalpy is the key. 

1.0Electron Gain Enthalpy Definition

Electron gain enthalpy is referred to as the amount of heat released by an atom when an electron is added to an isolated neutral atom in the gas phase to form a negative ion. In simple words, it is the enthalpy change which includes heat and work. It describes the ability of an atom and how strongly it can attract and gain electrons to form negative ions. The process can either release energy (negative EGH) or absorb energy (Positive EGH). This enthalpy change is related to the addition of an electron in an atom, which can also be expressed as: 

$X(g)+e^{-}\to X^{-}(g)+\Delta H$

Here, 

• X(g) is the Atom in the gas phase 
• e⁻ denote Electron
• ΔH is the Electron gain enthalpy (energy released or absorbed)
• The SI unit for electron gain enthalpy is KJ/mol.

2.0Electron Gain Enthalpy Values

The electron gain enthalpy values can either be negative or positive, which depends upon the following reactions:  

1. Negative Values (Exothermic Process): The exothermic process occurs when an atom readily accepts an electron, resulting in the release of energy. The electron gain enthalpy will be negative in such cases. For instance, fluorine (F) and chlorine (Cl) are one electron short of completing their octet, which results in them having high negative electron gain enthalpies. 
2. Positive Values (Endothermic Process): This is the case when the octet of an atom is full, and the addition of an electron requires energy to overcome the repulsion between the incoming electron and the already existing electrons in the outer shell of an atom. This results in a positive electron gain enthalpy. This generally happens if the electron configuration of an atom is stable, like noble gases such as neon. 

3.0Factors Affecting Electron Gain Enthalpy

The electron gain enthalpy is influenced by several factors that determine how an atom interacts with an incoming electron. These factors include:

Atomic Size

Atoms with smaller atomic radii generally have higher negative electron gain enthalpy values compared to their counterparts. The reason for this is the outermost electrons, which are closer to the nucleus, leading to a strong attractive force between the nucleus. Hence, the incoming electron needed a much stronger force. On the other hand, in larger atoms, the outermost electrons are farther from the nucleus. 

Effective Nuclear Charge

The Effective Nuclear Charge is the net positive charge experienced by an electron in an atom coming from both the attraction from the protons in the nucleus and repulsion from the other electrons of the atom. A higher Effective Nuclear Charge leads to a stronger attraction, making it easier for the atom to accept an electron, which results in a more negative electron gain enthalpy. At the same time, a lower Effective Nuclear Charge leads to a less negative or positive EGH. 

Shielding Effect

The repulsion between electrons in the inner and outer shells is known as the shielding effect. The outermost electrons are protected from the nucleus's full attraction as more inner electron shells are added (going down a group). As a result, the outermost electron experiences a less effective nuclear charge, which makes it harder for the atom to draw in another electron.

Electron Configuration

Atoms with filled or half-filled shells may experience repulsion and have less negative or positive EGH, whereas atoms that are close to achieving a full outer shell (such as halogens) tend to have more negative EGH. For example, with only one electron missing in their outer shell, halogens (such as fluorine and chlorine) are more likely to accept an electron and complete their valence shell, releasing a significant amount of energy. 

Electron-Electron Repulsion

When the valence shell of an atom already contains one or more electrons, the incoming electrons and the existing electrons repel each other. An electron gain enthalpy that is either positive or less negative can result from this repulsive force, which can make the process of gaining an electron less favourable.

4.0Variations on Electron Gain Enthalpy

Electron gain enthalpy varies in the periodic table across seven periods and eighteen groups:  

1. Across a Period: Moving from left to right across a period in the periodic table generally results in a more negative electron gain enthalpy due to the increase in the effective nuclear charge of an atom from left to right in a certain period. Conversely, electron gain enthalpy decreases if we move from right to left. 
2. Down or up a Group: As we move down a group, the electron gain enthalpy becomes more positive or less negative due to an increase in the size of an atom and the shielding effect, which makes it harder for the nucleus of an atom to attract more electrons. 

5.0Electron Gain Enthalpy and Electron Affinity

Electron Gain Enthalpy and Electron Affinity are two closely related concepts with slight differences and similarities. Electron affinity is a process which focuses on the energy change due to heat only in comparison to electron enthalpy which also focuses on energy change but involves both heat and work. The electron gain enthalpy and electron affinity share the following mathematical relationship: 

$\text{Electron gain enthalpy}=\text{electron affinity}-\frac{5}{2}RT$

Where, 

• R = universal gas constant 
• T = temperature in kelvin scale