Electron Gain Enthalpy

Introduction

Electron gain enthalpy is the energy change process during the addition of an electron to an isolated atom. The ability to add an electron and form an anion refers to electron affinity. The elements undergoing this energy change or addition of electrons are due to acquiring stable electronic configuration. Here, Chlorine undergoes the addition of an electron for a stable octet.

The electron affinities and negative ions or anions mainly belong to groups 6th and 7th of the periodic table. It has either negative or positive values depending on the size and nuclear charge of the elements. For example

Group	Elements	Electron affinities
6th	Sulphur	-200 KJ/mol
7th	Chlorine	-349 KJ/mol

From the above data, among chlorine and sulphur, chlorine has more negative electron gain enthalpy as it releases maximum energy. On the other hand, sulphur has negative electron gain enthalpy but less than chlorine. This is because chlorine requires one electron to achieve noble gas configuration whereas sulphur needs two electrons. Though chlorine readily adds an electron to achieve stability, it releases more energy and becomes more negative as compared to sulphur which adds two electrons. Elements also possess positive electron gain enthalpy only when a large amount of energy is required to add an electron.

What is Electron Gain Enthalpy of Elements?

It is referred to as the enthalpy or energy change when it gains an electron to form an anion. It is the amount of energy released or absorbed when an electron is added to an isolated gaseous atom. It is denoted by Δ_egH. The more the amount of energy is released, the more the electron gain enthalpy of an element. Among Fluorine and Chlorine, chlorine has a higher negative electron gain enthalpy value. This is because chlorine's outermost shell has a large space for the incoming electrons or addition of electrons and releases maximum energy.

Factors that Affect Electron Gain Enthalpy

The various factors that affect electron gain enthalpy of elements are −

Atomic size

The electron gain enthalpy decreases with an increase in atomic size. The large size of an atom decreases the nucleus's force of attraction for the incoming electron as the distance between the nucleus and the outermost shell increases.

Electronic configuration

The stable configuration elements have half-filled and fully filled subshells. The electron gain enthalpy will be higher for those elements which need to add electrons to their outermost shell to achieve maximum stability.

Effective nuclear charge

Electron gain enthalpy is directly dependent on the nuclear charge. The effective nuclear charge of the atom is the amount of total positive charge experienced by the incoming electron. With the increase in effective nuclear charge, electrons gain enthalpy also increases.

Electron Gain Enthalpy in Period

Factors	Trends in the period	Reasons
Electron gain enthalpy due to atomic size.	It becomes more and more negative.	Across periods, size decreases with the increase of electrons in the same shell, therefore, the force of attraction for the outermost electron increases due to a decrease in atomic size. Hence, electron gain enthalpy also increases (negatively).
Electron gain enthalpy due to effective nuclear charge.	It becomes more negative.	Across periods, the nuclear charge increases therefore the force of attraction for the addition of an electron also increases. Hence, electron gain enthalpy also increases (negatively).

Electron Gain Enthalpy Group

Factors	Trends in group	Reasons
Electron gain enthalpy due to atomic size.	From up to down, it becomes less negative.	Down the group, the atomic size of the element increases as the addition of new shell increases. Therefore, the distance between the nucleus and the last shell will also increase. This decreases the force of attraction for the incoming electron. Hence electron enthalpy decreases or is less negative.
Electron gain enthalpy due to effective nuclear charge.	From up to down, it becomes less negative.	Down the group, the effective nuclear charge of the element decreases due to an increased screening effect. Hence electron enthalpy decreases or is less negative.

Measurement and Use of Electron Affinity

Electron affinity is a quantitative method to measure how easily an electron is added to a neutral atom, thereby forming a negatively charged ion and, hence releasing energy. It is applicable for gaseous atoms only, as solids and liquids state change their energy level due to contact with other molecules and atoms.

$$\mathrm{Cl + e^{- }\rightarrow Cl^{- }+ energy }$$

• During the chemical reaction or process, the energy released is expressed in a negative value. It is also known as the exothermic process. It is represented by the symbol E_ea or EA. Unit is expressed in kJ/mol. Therefore, its measurement-

• First electron affinity − When a neutral atom adds the first electron it releases energy, and its first value is negative.

  $$\mathrm{O(g) + e^{-}\rightarrow O^-+ energy \:\:\:Ist EA= -142 kJ/mol.}$$

• Second electron affinity − When a negatively charged ion adds another electron, it requires a large amount of energy. The release of energy during attachment is overcome by adding new electrons, therefore second electron affinity becomes less negative or positive.

  $$\mathrm{O_g^- + e^{- }\rightarrow O^{2-} (g)+ energy , \:\:\: 2nd EA= +844 kJ/mol.}$$

  Positive electron affinity is due to the second electron added and the release of large amounts of energy. For oxygen, the second electron affinity is high as large forces into small and electron-dense spaces are applied.

• Electron affinity for elements by adding one electron to the outermost shell has negative values or is less negative as down in the group the size of an element increases, hence more energy is required to add an electron, making E_ea less negative. Here is the data −

Elements	Electron affinities (kJ/mol)
Fluorine	-333
Chlorine	-348
Bromine	-324
Iodine	-295
Astatine	-270.1

Use of electron affinity

• It helps identify molecules or atoms that are electron acceptors or donors.

• It helps identify reactants participating in charge transfer reactions.

• It helps identify the chemical hardness of the molecules.

• It helps identify the electronic chemical potential.

One-Electron Reduction

The reduction process is the gain of electrons or the addition of electrons to form a negative charge on elements. The atom that acquired electrons is said to be reduced by one electron.

Conclusion

The electron gain enthalpy is the release of energy when an extra electron is added to a neutral atom. On the other hand, electron affinity is the tendency of an element to add an electron to form an anion. In the addition of an electron or first electron enthalpy, the amount of energy released to obtain stability has a negative value. The more energy released, the more the electron gain enthalpy or negative value. However, during the addition of a second electron to an anion, the repulsive forces present due to a higher number of electrons results in a large amount of energy requirement and make the value less negative or positive.

FAQs

1. Why does electron gain enthalpy positive for metals?

Metals tend to lose electrons to obtain a stable configuration. Therefore, a large amount of energy or external energy is needed to add an electron, hence their electron gain enthalpy will be positive.

2. Why is the energy released in electron gain enthalpy?

There is a release of energy when an electron is added to a neutral or isolated gaseous atom to form an anion. Adding an electron to the neutral atom gives it stability, this addition of electrons releases energy.

3. Why does electron gain enthalpy positive for noble gases?

Noble gases have a stable configuration, and their outermost shell is fully occupied. The addition of electrons requires lots of energy, therefore electron gain enthalpy becomes positive.

4. Is electron affinity a physical or chemical property?

The addition of electrons in an isolated element is a physical property that influences the chemical properties.

5. Is electron affinity exothermic?

During the addition of electrons, the element releases a large amount of energy to obtain stability. This release of energy is called an exothermic process. Hence, it is exothermic.