Ionization & Electron Gain Enthalpy
Master the energetics of the Periodic Table. Conquer the critical exceptions of Ionization Enthalpy, understand positive Electron Gain Enthalpy, and decipher Electronegativity trends.
Module Focus
General trends like "increases across a period" and "decreases down a group" are too basic for NEET. Examiners love testing the anomalies caused by exactly half-filled or fully-filled orbitals, the penetration effect, and inter-electronic repulsions in compact atoms. This module strictly focuses on the exceptions that will dictate your rank.
1. Ionization Enthalpy ($\Delta_{\text{i}}H$)
Ionization Enthalpy is the minimum amount of energy required to remove the most loosely bound electron from an isolated gaseous atom in its ground state.
Successive Ionization Enthalpies
Energy is required to remove electrons one by one. The second ionization enthalpy ($\Delta_{\text{i}}H_2$) is the energy needed to remove an electron from a uni-positive ion.
$X^+(g) \xrightarrow{\Delta_{\text{i}}H_2} X^{2+}(g) + e^-$
Reason: It becomes increasingly difficult to remove an electron from a positively charged ion because the Effective Nuclear Charge ($Z_{\text{effective}}$) per electron drastically increases.
The general trend across Period 2 (Li to Ne) is that Ionization Enthalpy increases due to an increase in $Z_{\text{effective}}$. However, there are two massive exceptions:
Expected: B > Be
Actual: $\mathbf{Be (899 \text{ kJ/mol}) > B (801 \text{ kJ/mol})}$
Reason (Penetration Effect): In Be ($1s^2 2s^2$), the electron is removed from a highly penetrating, fully-filled 2s orbital. In B ($1s^2 2s^2 2p^1$), the electron is removed from the higher energy, less penetrating 2p orbital, which is easier.
Expected: O > N
Actual: $\mathbf{N (1402 \text{ kJ/mol}) > O (1314 \text{ kJ/mol})}$
Reason (Symmetry): Nitrogen has an exactly half-filled $2p^3$ subshell, which is exceptionally stable due to exchange energy. Oxygen ($2p^4$) readily loses one electron to achieve this stable half-filled configuration.
2. Electron Gain Enthalpy ($\Delta_{\text{eg}}H$)
When an electron is added to a neutral gaseous atom to convert it into a negative ion, the enthalpy change accompanying the process is defined as the Electron Gain Enthalpy.
For most elements, energy is released when an electron is added. Halogens (Group 17) have the most highly negative electron gain enthalpies because they are just one electron short of a stable noble gas configuration.
More negative value = Higher tendency to gain electron.
Energy must be supplied to add an electron to certain stable configurations.
- Noble Gases: Highly positive $\Delta_{\text{eg}}H$ because the electron must enter the next higher principal quantum level, which is highly unstable.
- Alkaline Earth Metals (Be, Mg): Positive $\Delta_{\text{eg}}H$ due to fully-filled $ns^2$ configuration.
The Ultimate Trap: Chlorine vs. Fluorine
We expect Fluorine to have the highest negative electron gain enthalpy because it is the most electronegative and smallest halogen.
Reason: Fluorine is extremely small. The electrons in its compact 2p subshell experience significant inter-electronic repulsions. Therefore, the incoming electron faces severe resistance. Chlorine has a larger 3p subshell, so the added electron suffers much less repulsion.
3. Electronegativity (EN)
Electronegativity is a qualitative measure of the ability of an atom in a chemical compound to attract shared electrons (bond pair) to itself. Unlike Ionization Enthalpy and Electron Gain Enthalpy, Electronegativity is not a measurable energy quantity; it is an arbitrary scale.
Pauling Scale & Trends
- Fluorine ($F$) is assigned an arbitrary value of 4.0, making it the most electronegative element.
- Oxygen ($O$) is second at 3.5, followed by Nitrogen ($N$) and Chlorine ($Cl$) at roughly 3.0.
- Trend: Increases across a period (as size decreases and $Z_{\text{effective}}$ increases). Decreases down a group (as size increases).
Electron Gain Enthalpy deals with an isolated gaseous atom gaining a free electron. Electronegativity deals with an atom within a molecule attracting a shared pair of electrons.
NEET Grand Test: Enthalpy Trends
15 High-Order Thinking Questions testing exceptions, successive jumps, and isoelectronic series.
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