Electron Gain Enthalpy

Understanding how atoms gain or lose energy is important in chemistry. One such concept is electron gain enthalpy. This topic helps explain why some elements easily accept electrons and others do not. In this article, we will learn what electron gain enthalpy means, how it works, the trends in the periodic table, and how it relates to electron affinity. We’ll also look at examples and comparisons across different elements and groups.

What is Electron Gain Enthalpy?

Electron gain enthalpy is the amount of energy released or absorbed when a neutral gaseous atom gains an extra electron to form a negative ion. It is expressed in kilojoules per mole (kJ/mol).

• If energy is released, the electron gain enthalpy is negative (exothermic process).
• If energy is absorbed, the electron gain enthalpy is positive (endothermic process).

Example: Chlorine gains one electron and releases -349 kJ/mol of energy. So, its electron gain enthalpy is negative.

Do Check: Acetaldehyde

Electron Gain Enthalpy Trend in the Periodic Table

The trend of electron gain enthalpy across the periodic table helps us predict which elements are more likely to gain electrons.

Why These Trends Happen?

• Across a Period: Atomic size decreases, and nuclear charge increases. So, atoms attract incoming electrons more strongly.
• Down a Group: Atomic size increases, and the outer shell is farther from the nucleus. The added electron experiences weaker attraction.

Do Check: Isomeris

Electron Gain Enthalpy Positive or Negative?

Electron gain enthalpy can be positive or negative depending on the atom’s willingness to accept an electron.

Electron Gain Enthalpy and Electron Affinity

While often used interchangeably, electron gain enthalpy and electron affinity are slightly different:

Note: A higher electron affinity usually means a more negative electron gain enthalpy.

Electron Gain Enthalpy Down the Group

As we move down a group in the periodic table (like from Fluorine to Iodine in group 17), the electron gain enthalpy becomes less negative.

• Reason: Atomic size increases and the attraction between the nucleus and the added electron becomes weaker.
• Shielding effect also plays a role. Inner electrons block the attraction from the nucleus, making it harder to hold on to the new electron.

Do Check: Reactivity Series

Electron Gain Enthalpy of Halogens

Halogens (Group 17) have the most negative electron gain enthalpies in the periodic table. This means they easily accept electrons.

Chlorine has the most negative value among them due to a balance of small size and low electron repulsion.

Electron Gain Enthalpy Order (Comparative)

Below is the order of electron gain enthalpy from most negative (releases most energy) to least negative or positive:

Cl > F > Br > I > O > S > N > Ne

• Halogens top the list due to high electronegativity.
• Noble gases like Neon have positive values as they resist adding electrons.

Factors Affecting Electron Gain Enthalpy

Several factors decide how easily an atom accepts an electron:

1. Atomic Size – Smaller atoms have more negative electron gain enthalpy.
2. Nuclear Charge – More protons = stronger attraction = more negative value.
3. Electron Configuration – Stable configurations (like noble gases) resist change.
4. Shielding Effect – More inner electrons = weaker pull on added electron.

Conclusion

Electron gain enthalpy is a key concept in understanding chemical bonding and reactivity. It explains why non-metals are good at gaining electrons and why noble gases resist it. Understanding the trends and examples makes it easier to predict chemical behavior, especially when studying periodic properties, ion formation, and electronegativity.