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Grade 10-11 (age 15-17)

high school Electronegativity & Polarity

Electronegativity is how strongly an atom in a covalent bond pulls on the shared electron pair, and it increases toward the upper right of the periodic table. If two atoms have the same electronegativity, the pair sits in the middle and the bond is nonpolar covalent; if they differ, the pair shifts to one side and the bond is polar covalent. Even with polar bonds, a molecule whose shape is symmetric can have its polarities cancel and be nonpolar. In the picture, increasing the electronegativity difference moves the shared pair toward B, and the partial charges δ+ and δ− on the two sides become clearer.

Who Pulls the Electron Pair Harder?

1.5
👀 Reading the Picture
①Electronegativity is how strongly an atom pulls on a shared electron pair
②When ΔEN = 0 the pair sits exactly between the two atoms, so the bond is nonpolar
③As ΔEN grows, the pair shifts toward B, the more electronegative atom, giving B a δ− and A a δ+

Electronegativity and Bond Polarity

Electronegativity Trend
Increases toward the upper right of the periodic table (F is the largest)
Electronegativity increases to the right across a period and upward in a group
Bond Polarity
ΔEN = 0 → nonpolar covalent bond, ΔEN > 0 → polar covalent bond
The larger the electronegativity difference, the more polar the bond (the larger the partial charges)

Molecular Polarity: Shape Matters

🧭 Bond Polarity + Molecular Shape
①Even with polar bonds, a molecule with a symmetric shape has its polarities cancel and is nonpolar
②CO₂ is linear and symmetric, so it is a nonpolar molecule
③H₂O is bent and not symmetric, so it is a polar molecule

Reason It Out

Example 1
Explain the partial-charge distribution and bond polarity in HCl.
1
Cl is more electronegative than H.
2
So the shared electron pair shifts toward Cl.
Cl: δ−, H: δ+ ⇒ polar covalent bond
▸ A polar covalent bond with δ− on Cl and δ+ on H
The more electronegative atom always carries the δ−, just as B does in the picture.
Example 2
CO₂ and H₂O both have polar bonds, so why is their molecular polarity different?
1
In both molecules the bonds themselves are polar.
2
The shape of the molecule decides whether the polarities cancel.
CO₂ (linear, symmetric) → cancels → nonpolar / H₂O (bent) → polar
▸ CO₂ is nonpolar and H₂O is polar (a difference in shape)
Even when the bonds are polar, a molecule with a symmetric shape is nonpolar.

Summary

Key Result
Bond polarity = ΔEN, molecular polarity = bond polarity + molecular shape (symmetry)
The electronegativity difference sets the bond polarity, and adding the molecular shape decides the polarity of the molecule
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
1
Look at the shape of each molecule. CO₂ is linear and symmetric.
2
Because it is symmetric, the polarities of the two C=O bonds cancel.
CO₂: linear and symmetric ⇒ nonpolar molecule
🎯 Exam Points
①Electronegativity increases toward the upper right of the periodic table (F is the largest)
②ΔEN = 0 gives a nonpolar bond and ΔEN > 0 a polar bond
③The more electronegative atom carries δ−
④Molecular polarity depends on bond polarity and molecular shape, and symmetry makes the polarities cancel
⑤The comparison of nonpolar CO₂ with polar H₂O comes up often
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Molecular Structure
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Chemical Equations and Stoichiometry

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