What is the Difference Between Electron Affinity and Electron Gain Enthalpy

The main difference between electron affinity and electron gain enthalpy is that electron affinity is the energy change associated with the addition of an electron to a gaseous atom to form a negative ion, whereas electron gain enthalpy is the enthalpy change associated with the addition of one mole of electrons to one mole of gaseous atoms to form one mole of negative ions.

Electron affinity and electron gain enthalpy are key concepts in chemistry. Both describe the energy changes associated with the addition of an electron to an isolated gaseous atom. Both measurements provide insights into the stability and reactivity of atoms.

Key Areas Covered

1. What is Electron Affinity 
      – Definition, Features 
2. What is Electron Gain Enthalpy
      – Definition, Features 
3. Similarities Between Electron Affinity and Electron Gain Enthalpy
      – Outline of Common Features
4. Difference Between Electron Affinity and Electron Gain Enthalpy
      – Comparison of Key Differences
5. FAQ: Electron Affinity and Electron Gain Enthalpy
      – Frequently Asked Questions

Key Terms

Electron Affinity, Electron Gain Enthalpy

What is Electron Affinity

Electron affinity is defined as the energy change associated with the addition of an electron to a gaseous atom to form a negative ion. The following equation can represent this process:

X(g) + e⁻ → X⁻(g)

Here, X(g) represents a gaseous atom of an element, e⁻ is an electron, and X⁻(g) is a negative ion. The measurement of electron affinity involves experimental determination of the energy change that occurs when an electron is added to an isolated gaseous atom. The process can either release energy (exothermic) or absorb energy (endothermic), leading to negative or positive values of electron affinity, respectively.

A negative electron affinity indicates that the process is energetically favorable, as there is an energy release during electron addition. Conversely, a positive electron affinity suggests that the process is energetically unfavorable, requiring an input of energy.

Electron affinity is a key determinant of an element’s reactivity. Elements with high electron affinity are more likely to form negative ions, contributing to their chemical reactivity. Moreover, in the formation of ionic compounds, electron affinity plays a crucial role. Elements with a strong tendency to accept electrons are likely to form negative ions, combining with positive ions to create a stable ionic lattice.

Furthermore, the periodic trend of electron affinity across the periodic table provides valuable information about the electron-attracting ability of elements in different periods and groups.

What is Electron Gain Enthalpy

Electron gain enthalpy (ΔHeg​) is defined as the enthalpy change associated with the process of adding one mole of electrons to one mole of gaseous atoms, resulting in the formation of one mole of negative ions.

Experimentally determining electron gain enthalpy involves measuring the energy change accompanying the addition of an electron to an isolated gaseous atom. The process can be either exothermic or endothermic, leading to negative or positive values of ΔHeg, respectively.

For example, if the electron addition releases energy (an exothermic process), ΔHeg​ is negative. This indicates that the atom is more stable in the form of a negative ion. On the other hand, if energy is absorbed during electron addition (an endothermic process), ΔHeg​ is positive, signifying that the atom is less stable as a negative ion.

Factors Influencing Electron Gain Enthplay

Several factors influence the magnitude and sign of electron gain enthalpy:

Atomic Size: Generally, electron gain enthalpy becomes more negative as you move across a period from left to right. This is because atoms on the right side of the periodic table have a smaller size and higher effective nuclear charge, making the addition of an electron more favorable.

Subshell Configuration: The stability of half-filled or completely filled subshells affects electron gain enthalpy. Atoms with a half-filled or completely filled subshell tend to have more favorable ΔHeg​ values.

Shielding Effect: The shielding effect of inner electrons plays a role. The more effective the shielding, the less the outer electron is attracted to the nucleus, making electron gain enthalpy less negative.

Electron gain enthalpy is a key factor in determining the reactivity of elements. In fact, elements with a high tendency to gain electrons (more negative ΔHeg​) are more likely to form negative ions. Moreover, in the formation of ionic compounds, electron gain enthalpy is a crucial factor. Additionally, elements with a strong tendency to gain electrons are likely to form negative ions, combining with positive ions to create a stable ionic lattice. 

Similarities Between Electron Affinity and Electron Gain Enthalpy

• Electron affinity and electron gain enthalpy focus on the energy change associated with adding an electron to a neutral gaseous atom to form a negative ion.
• Both concepts can have either positive or negative values.

Difference Between Electron Affinity and Electron Gain Enthalpy

Definition

Electron affinity is the energy change associated with the addition of an electron to a gaseous atom to form a negative ion. Electron gain enthalpy is specifically the enthalpy change associated with the addition of one mole of electrons to one mole of gaseous atoms to form one mole of negative ions.

Measurement

Electron affinity can be measured both in the gas phase and in condensed phases. It is a more general term that doesn’t necessarily involve constant pressure conditions. However, the term “enthalpy” implies that the measurement of electron gain enthalpy is typically done under constant pressure conditions, reflecting the enthalpic changes in the system.

Units

Moreover, electron affinity is expressed in units of energy per mole, such as kilojoules per mole (kJ/mol) or electron volts (eV). Electron gain enthalpy is also expressed in units of energy per mole but specifically as enthalpy change, typically measured in kilojoules per mole (kJ/mol).

FAQ: Electron Affinity and Electron Gain Enthalpy

Is electron gain enthalpy always positive?

Can electron affinity be zero?

Noble gases have zero electron affinity.

Why is electron affinity exothermic?

Electron affinity is exothermic because energy is released when an electron is added to an atom, resulting in a more stable, lower-energy state.

Conclusion

Electron affinity is the energy change associated with the addition of an electron to a gaseous atom to form a negative ion. Meanwhile, electron gain enthalpy is the enthalpy change associated with the addition of one mole of electrons to one mole of gaseous atoms to form one mole of negative ions. Thus, this is the main difference between electron affinity and electron gain enthalpy.

Reference:

1. “Electron Gain Enthalpy.” Byju’s.
2. “Electron Affinity.” LibreTexts Chemistry.

Image Courtesy:

1. “Electron affinities of the elements” By Sandbh – Own work (CC BY-SA 3.0) via Commons Wikimedia