Ionization Enthalpy vs Potential
Periodic Trends & Exceptions for JEE & NEET
1. The Definitions: Are They the Same?
While often used interchangeably, there is a subtle thermodynamic difference between these terms.
Ionization Energy (IE)
The minimum energy required to remove the most loosely bound electron from an isolated gaseous atom ($M$) to form a gaseous cation ($M^+$).
Equation: $M(g) + IE \rightarrow M^+(g) + e^-$
Unit: Electron-volts per atom ($eV/atom$) or $kJ/mol$.
| Term | Symbol | Context |
|---|---|---|
| Ionization Energy | $IE$ | General term for the energy required. |
| Ionization Enthalpy | $\Delta_i H$ | Thermodynamic quantity measured at constant pressure. Unit: $kJ \cdot mol^{-1}$. |
| Ionization Potential | $IP$ | The potential difference (in Volts) required to remove the electron. Numerically, $1\,eV \approx 96.49\,kJ/mol$. |
2. General Periodic Trends
Across a Period (Left $\rightarrow$ Right):
- $\Delta_i H$ Increases.
- Reason: Atomic size decreases and Effective Nuclear Charge ($Z_{eff}$) increases. The nucleus holds electrons more tightly.
Down a Group (Top $\rightarrow$ Bottom):
- $\Delta_i H$ Decreases.
- Reason: Atomic size increases (new shells added) and Shielding Effect increases, making outer electrons easier to remove.
3. Crucial Exceptions (Must-Know for Exams)
Exceptions arise due to Penetration Effect and Electronic Configuration Stability.
Expected: $B > Be$ (since B is smaller)
Actual: $Be > B$
Reason (Penetration Effect):
Be Configuration: $1s^2 2s^2$ (Full s-orbital)
B Configuration: $1s^2 2s^2 2p^1$
It is harder to remove an electron from the $2s$ orbital (more penetrating towards nucleus) than the $2p$ orbital.
Expected: $O > N$ (since O is smaller)
Actual: $N > O$
Reason (Half-filled Stability):
N Configuration: $2p^3$ (Stable half-filled)
O Configuration: $2p^4$
Oxygen loses an electron easily to achieve the stable $2p^3$ half-filled state.
The trend is zigzag: $B > Tl > Ga > Al > In$
Key Anomaly: $Ga > Al$
Reason: Transition Contraction (d-block contraction). Gallium has 3d electrons which provide poor shielding, increasing $Z_{eff}$ on outer electrons.
Practice Quiz (10 Questions)
Test your understanding of the concepts above.
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