Electronegativity — Pauling & Allred-Rochow Scales

CompleteFundamentals of Chemistry

Electronegativity is the power of an atom in a molecule to attract shared electrons; two important quantitative scales exist.

Full notes

Definition: Electronegativity: the tendency of an atom in a molecule to attract the shared pair of electrons towards itself. Unlike ionization enthalpy, it applies to atoms in bonded states.

1. Pauling's scale

Based on bond energies: |χA − χB| = 0.102 √(Δ), where Δ = (observed bond energy A–B) − arithmetic mean of A–A and B–B bond energies (with corrections). F is set at 4.0.

2. Allred–Rochow scale

Electronegativity ∝ force felt by an electron at the covalent radius: χ = 0.359 (Z*/r²) + 0.744, where Z* is Slater's effective nuclear charge and r the covalent radius in Å.

3. Trends & uses

Increases across a period (Li 1.0 → F 4.0).
Decreases down a group (F 4.0 > Cl 3.0 > Br 2.8 > I 2.5).
Explains bond polarity and % ionic character.
Electronegativity of an element is NOT a constant — depends on oxidation state/coordination.

Note: Exam point: compare Pauling (thermochemical, relative, F=4.0) vs Allred-Rochow (based on Z*/r², directly calculable from Slater rules).

Exam notes

Pauling: bond energies, F = 4.0, (χA−χB) = 0.102√Δ. Allred-Rochow: χ = 0.359 Z*/r² + 0.744. Trends: ↑ period, ↓ group.

Important points

Electronegativity values help predict bond polarity and percentage ionic character.

Common mistakes

Confusing electronegativity (bonded atom) with electron affinity (isolated atom).

PreviousAtomic & Ionic Radii Trends Next Ionization Enthalpy & Electron Gain Enthalpy