Electron Affinity 

Electron affinity is a concept in chemistry that describes the energy change when an electron is added to a neutral atom in the gaseous state to form a negative ion. It provides insight into how atoms gain electrons and form anions. Let’s understand Electron affinity in detail.

1.0Definition of Electron Affinity

Electron affinity definition involves the amount of energy released when a gaseous atom gains an electron to form a gaseous anion. It is usually expressed in kilojoules per mole (kJ/mol).

                               X(g) + e⁻ → X⁻(g) + ΔE

Here, ΔE represents the electron affinity, which is typically negative because energy is released when the atom gains an electron.

2.0First and Second Electron Affinity

First Electron Affinity (EA₁): 

The energy changes when one electron is added to a neutral atom in the gaseous state. It is usually exothermic (negative value) because energy is released as the electron is attracted to the nucleus.

Cl (g) +e⁻ → Cl−(g)   ΔH =−349 kJ/mol

Second Electron Affinity (EA₂): 

The energy change when a second electron is added to a negatively charged ion. This process is usually endothermic (positive value) because energy is required to overcome the repulsion between the negatively charged ion and the incoming electron.

         O⁻(g) + e⁻ → O₂⁻(g)  ΔH = +844 kJ/mol

3.0Trends in Electron Affinity

• Across a Period: Electron affinity generally becomes more negative (more exothermic) across a period from left to right. This is because the atomic size decreases, and the effective nuclear charge increases, making it easier for the nucleus to attract additional electrons.
• Down a Group: Electron affinity becomes less negative (less exothermic or even endothermic) as you move down a group. This is due to the increasing atomic size and shielding effect, which reduce the attraction between the nucleus and the added electron.

4.0Factors Affecting Electron Affinity

• Nuclear Charge: 

A higher effective nuclear charge increases the attraction between the nucleus and the added electron, making the electron affinity more negative.

• Atomic Size: 

Smaller atoms have electrons closer to the nucleus, resulting in a stronger attraction for an additional electron, hence a more negative electron affinity.

• Electron Configuration: 

Atoms with stable electron configurations (like noble gases or half-filled p orbitals) have less tendency to gain electrons, leading to a lower (less negative or positive) electron affinity. Conversely, atoms that gain electrons to achieve a stable configuration have a higher (more negative) electron affinity.

• Shielding Effect: 

Increased shielding by inner electrons reduces the effective nuclear charge felt by the added electron, making the electron affinity less negative.

5.0Exceptions to General Trends

• Group 2 Elements: 

Alkaline earth metals have relatively low electron affinities because adding an electron requires starting a new p orbital, which is less favourable.

• Group 15 Elements: 

The p orbitals in these elements are half-filled, and adding another electron introduces repulsion in the p orbital, making electron affinity less negative.

• Noble Gases: 

These elements have a positive electron affinity because their electron configuration is already stable, and adding an electron would require energy input to place it in a new shell or orbital.

6.0Difference between Electron Affinity and Electronegativity

7.0Difference between Electron Affinity and Ionization Energy

8.0Applications of Electron Affinity

• Predicting Reactivity: 

Elements with high (more negative) electron affinities tend to be more reactive non-metals, such as halogens, which readily gain electrons to form anions.

• Chemical Bonding:

Electron affinity values help in understanding bond formation, particularly in ionic compounds where electron transfer is involved.

• Material Science and Catalysis: 

Understanding electron affinities is crucial in designing materials and catalysts, particularly those involving redox reactions.

9.0Measurement of Electron Affinity

• Experimental Methods: Electron affinities are often measured using spectroscopic methods or calculated using thermodynamic cycles like the Born-Haber cycle in ionic compounds.