Electronegativity & Polarity

Who Pulls the Electrons Harder

Electronegativity difference ΔEN1.5

👀 See It

①Electronegativity is how strongly an atom pulls the shared electron pair

②Equal electronegativity keeps the pair centered — nonpolar

③A large difference shifts the pair to one side, making δ− (the puller) and δ+ (the other)

Electronegativity and Bond Polarity

Electronegativity trend

increases toward the upper right of the periodic table (F is the max)

Within a period it rises to the right; within a group, upward

Bond polarity

ΔEN = 0 → nonpolar covalent, ΔEN > 0 → polar covalent

The larger the electronegativity difference, the more polar the bond (partial charges)

Molecular Polarity — Shape Matters

🧭 Bond Polarity + Molecular Shape

①Even with polar bonds, a symmetric molecule cancels them out — nonpolar

②CO₂ is linear and symmetric → nonpolar molecule

③H₂O is bent (asymmetric) → polar molecule

Reason It Out

Example 1

Explain the partial-charge distribution and bond polarity in HCl.

Electronegativity is Cl > H.

The shared electron pair shifts toward Cl.

Cl: δ−, H: δ+ ⇒ polar covalent bond

▸ Polar covalent: δ− on Cl, δ+ on H

The more electronegative atom always becomes δ−.

Example 2

CO₂ and H₂O both have polar bonds, so why is their molecular polarity different?

Both molecules have polar bonds themselves.

Whether they cancel depends on molecular shape.

CO₂ (linear·symmetric) → cancels → nonpolar / H₂O (bent) → polar

▸ CO₂ nonpolar, H₂O polar (shape difference)

Even with polar bonds, a symmetric shape makes a nonpolar molecule.

Wrap-up

Key result

bond polarity = ΔEN, molecular polarity = bond polarity + shape (symmetry)

Bond polarity from ΔEN; add the shape to judge molecular polarity

2021 CSAT Science (Chemistry Ⅰ) type, adapted

Which of the following is a nonpolar molecule? (HF, H₂O, NH₃, CO₂, HCl)

①HF

②H₂O

③NH₃

④CO₂

⑤HCl

▸ ④ CO₂

Check each shape — CO₂ is linear and symmetric.

Symmetry cancels the bond polarities.

CO₂: linear symmetric ⇒ nonpolar molecule

🎯 Exam Points

①electronegativity: larger toward upper-right (F max)

②ΔEN=0 nonpolar bond, ΔEN>0 polar bond

③the more electronegative atom is δ−

④molecular polarity = bond polarity + shape (symmetry = cancellation)

⑤CO₂ nonpolar vs H₂O polar is the classic comparison

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