Electron Gain Enthalpy vs. Electron Affinity
The Complete Guide to Trends & Exceptions for JEE/NEET
1. Understanding the Core Difference
Many students confuse Electron Gain Enthalpy ($\Delta_{eg}H$) with Electron Affinity ($A_e$). While they describe the same process—adding an electron to an atom—their sign conventions are opposite.
Definition
Electron Gain Enthalpy ($\Delta_{eg}H$): The enthalpy change when an electron is added to a neutral gaseous atom ($X$) to convert it into a negative ion ($X^-$).
$$X(g) + e^- \rightarrow X^-(g)$$
| Feature | Electron Affinity ($A_e$) | Electron Gain Enthalpy ($\Delta_{eg}H$) |
|---|---|---|
| Nature | Measure of "love" for electron. | Thermodynamic energy change. |
| Sign Convention | Positive if energy is released. | Negative if energy is released (Exothermic). |
| Relation | $\Delta_{eg}H = -A_e - \frac{5}{2}RT$ (At absolute zero, $\Delta_{eg}H = -A_e$) |
|
2. General Periodic Trends
Across a Period (Left to Right)
Trend: $\Delta_{eg}H$ becomes more negative (more energy released).
Reason: Atomic size decreases and effective nuclear charge increases, attracting the incoming electron more strongly.
Down a Group (Top to Bottom)
Trend: $\Delta_{eg}H$ becomes less negative.
Reason: Atomic size increases. The added electron enters a shell further from the nucleus, reducing the attraction.
3. Critical Exceptions (The "Rank Deciders")
JEE and NEET exams specifically target these anomalies where general trends fail.
Exception 1: Halogens (Fluorine vs. Chlorine)
Expected: $F$ should be more negative than $Cl$ due to smaller size.
Actual: $Cl > F$ (Chlorine is more negative).
Reason: Fluorine is extremely small ($2p$ orbital). Adding an electron causes significant inter-electronic repulsion among the existing valence electrons. Chlorine has a larger $3p$ orbital, which accommodates the electron easily.
Order: $Cl > F > Br > I$
Exception 2: Group 16 (Oxygen vs. Sulphur)
Actual: $S > Se > Te > Po > O$
Anomaly: Oxygen has the least negative electron gain enthalpy in the group.
Reason: Similar to Fluorine, Oxygen's small size leads to high electron repulsion, making it less favorable to accept an electron compared to Sulphur.
Exception 3: Noble Gases & Group 2
Observation: They have Positive Electron Gain Enthalpy.
Reason:
- Noble Gases ($ns^2 np^6$): Stable octet. They resist adding an electron; energy must be supplied.
- Group 2 ($ns^2$) & Nitrogen ($np^3$): Stable fully-filled or half-filled orbitals make electron addition difficult (often positive or near zero).
Important Note: Second Electron Gain Enthalpy
While the first $\Delta_{eg}H$ can be negative (exothermic), the Second Electron Gain Enthalpy is ALWAYS Positive (Endothermic).
Why? You are adding a negative electron to an already negative ion ($O^- \rightarrow O^{2-}$). Strong electrostatic repulsion must be overcome by supplying energy.
Test Your Understanding
10 Questions | +4 Correct, -1 Wrong
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