🔹 Definition

Electron Gain Enthalpy is the enthalpy change when an electron is added to an isolated gaseous atom.

• Usually negative (energy released)
• More negative = greater tendency to gain electron

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🔹 Representation

$$X(g) + e^- \rightarrow X^-(g)$$

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🔹 General Trends

Across a Period (Left → Right)

EGE becomes more negative

Reason:

• Increase in nuclear charge → stronger attraction for incoming electron

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Down a Group (Top → Bottom)

EGE becomes less negative

Reason:

• Increase in size → electron added far from nucleus → less attraction

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IMPORTANT EXCEPTIONS (Very Important for JEE/NEET)

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❗ Exception 1: Be, Mg (Group 2)

EGE ≈ zero or slightly positive

Reason:

• Stable ns² configuration
• Incoming electron enters higher energy p-orbital

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❗ Exception 2: N (Group 15)

EGE is less negative than expected

Reason:

• Stable half-filled configuration (np³)
• Adding electron causes electron-electron repulsion

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❗ Exception 3: Noble Gases (Group 18)

EGE is positive

Reason:

• Completely filled orbitals → very stable
• Electron must enter new shell → requires energy

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❗ Exception 4: Fluorine vs Chlorine

👉 Cl has more negative EGE than F ❗

❌ Expected: F > Cl
✅ Actual: Cl > F

Reason:

• F is very small → strong electron-electron repulsion in 2p orbital
• Cl has larger size → less repulsion → easier electron addition

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🔹 Important Order Example

Among halogens:
Cl > F > Br > I

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Successive Electron Gain Enthalpy

👉 Adding 2nd electron:$$X^- + e^- \rightarrow X^{2-}$$

❗ Always positive
Reason:

• Electron added to already negative ion → strong repulsion

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🔹 Elements in Group 16

• Oxygen (O)
• Sulfur (S)
• Selenium (Se)
• Tellurium (Te)
• Polonium (Po)

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Increases down the group
O < S < Se < Te

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Why is the 2nd Electron Gain Enthalpy of Oxygen Positive?

Consider the process:$$\text{O}^- (g) + e^- \rightarrow \text{O}^{2-} (g)$$

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🔹 Key Reason: Electron–Electron Repulsion

After gaining one electron:

• Oxygen becomes O⁻ (negatively charged)

Now, adding another electron:

• Incoming electron is repelled by the already negative ion

✔ So, energy must be supplied → EGE becomes positive