What is electron gain enthalpy // what is electron gain enthalpy in Chemistry class 11

 

⚛️ Electron Gain Enthalpy (ΔegH)

Electron gain enthalpy is the amount of energy released or absorbed when an electron is added to an isolated gaseous atom to form a negative ion.

🔬 Definition:

"Electron gain enthalpy is the enthalpy change when one mole of electrons is added to one mole of isolated gaseous atoms."

📘 General Reaction:


X_{(g)} + e^- \rightarrow X^-_{(g)} \quad \Delta_{eg}H
  • = isolated gaseous atom
  • = anion formed
  • = added electron
  • = electron gain enthalpy

🔥 Key Points:

  • Negative electron gain enthalpy: Energy is released (exothermic). Most nonmetals show this behavior.
    Example: Chlorine

  Cl_{(g)} + e^- \rightarrow Cl^-_{(g)} \quad \Delta_{eg}H = -349 \text{ kJ/mol}
  • Positive electron gain enthalpy: Energy is absorbed (endothermic). Some elements (like noble gases or N) show this.
    Example: Nitrogen, Neon

📊 Trends in Periodic Table:

  • Across a Period (→):
    Electron gain enthalpy becomes more negative (more energy released) due to increasing nuclear charge.

  • Down a Group (↓):
    Becomes less negative (less energy released) due to increased atomic size and electron shielding.

🧠 Why Important?

  • Helps to understand reactivity of nonmetals
  • Explains formation of anions (e.g., Cl⁻, O²⁻)
  • Useful in calculating electron affinity and chemical behavior


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