Electronegativity
Concept, Trends & Applications | Periodic Properties
1. Definition
Electronegativity ($\chi$): The tendency of an atom in a chemical molecule to attract the shared pair of electrons towards itself.
Unlike Electron Gain Enthalpy (measured for isolated gaseous atoms), Electronegativity is not a measurable energy but a relative tendency.
2. Measuring Scales
A. Pauling Scale (Most Common)
Based on bond energies. Fluorine is assigned the highest value of 4.0.
$$ \chi_A - \chi_B = 0.208 \sqrt{\Delta} $$
Where $\Delta$ is resonance energy in kcal/mol.
B. Mulliken Scale
Based on Ionization Energy (IE) and Electron Affinity (EA).
$$ \chi_{M} = \frac{IE + EA}{2} $$
Relation: $\chi_{Pauling} \approx \frac{\chi_{Mulliken}}{2.8}$ (if energies are in eV).
3. Factors Affecting Electronegativity
- Atomic Size: EN $\propto \frac{1}{\text{Size}}$. Smaller atoms attract electrons more strongly.
- Effective Nuclear Charge ($Z_{eff}$): EN $\propto Z_{eff}$. Higher positive charge attracts electrons.
- Oxidation State: EN increases with positive oxidation state. Example: $Fe^{+3} > Fe^{+2}$.
- Hybridization: EN $\propto$ s-character. $sp (50\%) > sp^2 (33\%) > sp^3 (25\%)$.
(Carbon in Ethyne is more electronegative than in Ethane).
4. Periodic Trends
A. General Trends
| Direction | Trend | Reason |
|---|---|---|
| Period (L to R) | Increases | Size decreases, $Z_{eff}$ increases. |
| Group (Top to Bottom) | Decreases | Size increases significantly. |
B. Important Exceptions
- Inert Gases: Generally considered to have zero EN (stable octet), though Xenon forms compounds.
- Group 13 (The Zig-Zag): $B > Tl > In > Ga > Al$.
Reason: Poor shielding by d-electrons (in Ga) and f-electrons (in Tl) increases $Z_{eff}$, making Ga > Al. - Group 14: $C > Pb > Si \approx Ge \approx Sn$. Lead has higher EN due to Lanthanoid contraction.
Reference Values: $F(4.0) > O(3.5) > N(3.0) \approx Cl(3.0) > C(2.5) \approx S(2.5) \approx I(2.5) > H(2.1)$.
5. Applications
- Nature of Bond:
- Difference > 1.7 $\rightarrow$ Ionic
- Difference < 1.7 $\rightarrow$ Covalent
- Percentage Ionic Character (Haney-Smith Equation): $$ \% \text{ Ionic} = 16(\Delta \chi) + 3.5(\Delta \chi)^2 $$
- Bond Strength: Greater $\Delta \chi$ $\rightarrow$ Stronger bond.
- Metallic Character: Metals have low EN; Non-metals have high EN.
Practice Quiz
Test your knowledge on Electronegativity.
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