Electronegativity - Chemistry

1. Fundamental Concepts

Definition: The relative ability of an atom in a chemical compound to attract shared electrons to itself (dimensionless, relative value).
Core Idea: Higher electronegativity means stronger electron-attracting ability, making the atom more likely to form a negative oxidation state.
Reference Scale: Pauling scale (the most commonly used), where fluorine (F) has the highest electronegativity (3.98) and cesium (Cs) has the lowest (0.79).

2. Key Concepts

Periodic Trends
- Across a period (left to right): Electronegativity increases (due to increasing nuclear charge and decreasing atomic radius).
- Down a group (top to bottom): Electronegativity decreases (due to increasing atomic radius outweighing the effect of increased nuclear charge).
Bond Type Correlation
- Electronegativity difference (ΔEN) > 1.7: Ionic bond (e.g., NaCl: Na 0.93, Cl 3.16, ΔEN = 2.23).
- Electronegativity difference (ΔEN) < 1.7: Covalent bond
  - ΔEN = 0–0.4: Nonpolar covalent bond (e.g., O₂).
  - ΔEN = 0.4–1.7: Polar covalent bond (e.g., HCl).
Characteristics
- No absolute numerical value, only for relative comparison.
- Metals generally have electronegativity < 1.8; nonmetals generally have electronegativity > 1.8.

3. Examples

Easy

Question: Determine the bond type between hydrogen (H, 2.20) and oxygen (O, 3.44).
Analysis: Calculate the electronegativity difference: ΔEN = 3.44 - 2.20 = 1.24. Since 1.24 < 1.7, the bond is a polar covalent bond.

Medium

Question: Compare the electronegativity of carbon (C, 2.55), nitrogen (N, 3.04), and silicon (Si, 1.90), and arrange them in descending order.
Analysis: Apply periodic trends:

C and N are in the same period; N is to the right of C, so EN(N) > EN(C).
C and Si are in the same group; C is above Si, so EN(C) > EN(Si).

Conclusion: N > C > Si

Hard

Question: Explain why AlCl₃ (Al 1.61, Cl 3.16) is a covalent compound while NaCl (Na 0.93, Cl 3.16) is an ionic compound.Analysis:

Calculate electronegativity differences:
- Al-Cl: ΔEN = 3.16 - 1.61 = 1.55 (< 1.7)
- Na-Cl: ΔEN = 3.16 - 0.93 = 2.23 (> 1.7)
Consider ionic polarization: Al³⁺ has a small radius and high charge, leading to strong polarizing power. It distorts the electron cloud of Cl⁻, making the bond transition from ionic to covalent. Na⁺ has a larger radius and lower charge, with weak polarizing power, forming a typical ionic bond.

Conclusion: AlCl₃ is covalent due to small ΔEN and strong ionic polarization; NaCl is ionic due to large ΔEN and weak ionic polarization.

4. Problem-Solving Techniques

Trend Application
- Prioritize using "increase across a period, decrease down a group" to judge relative electronegativity, avoiding the trap of absolute numerical values.
Bond Type Judgment
- Use 1.7 as the critical value for ΔEN. Assist the judgment with ionic polarization (cations with high charge and small radius have strong polarizing power, leading to covalent bond tendency).
Oxidation State Inference
- Atoms with higher electronegativity exhibit negative oxidation states; those with lower electronegativity exhibit positive oxidation states (e.g., O is negative and H is positive in H₂O).