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1.3.6Physical Chemistry

Bond Polarity — Oxford AQA International A Level Chemistry Revision Notes & Topical Questions

5 min read 5 parts Lesson 19 of 63

📚 Oxford AQA International A Level Chemistry Hub / Topical Revision
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Part 1 of 5Electronegativity

1.3.6 Bond Polarity
Definition
  • Electronegativity is the power of an atom to attract the bonding pair of electrons in a covalent bond.
Scale
  • The Pauling scale runs from 0 to 4, with fluorine highest at 4.0, then oxygen, nitrogen and chlorine.
Across a period
  • Electronegativity rises because nuclear charge increases while shielding stays about the same.
Down a group
  • Electronegativity falls because the bonding pair is further from the nucleus and more shielded.

Part 2 of 5Polar Bonds

Polar Bonds
Unequal sharing
  • When two atoms of different electronegativity bond, the bonding pair is drawn closer to the more electronegative atom.
Partial charges
  • The atom with the greater pull becomes slightly negative, δ−, and the other slightly positive, δ+.
Non-polar bonds
  • Bonds between identical atoms, such as Cl–Cl, share electrons equally.
Continuum
  • The greater the difference in electronegativity, the more ionic the bonding character becomes.

Part 3 of 5Polar and Non-Polar Molecules

Dipole
  • A molecule is polar if it has an overall dipole, meaning the bond dipoles do not cancel out.
Symmetrical molecules
  • CO₂, CCl₄, BF₃ and SF₆ have polar bonds but are non-polar because their dipoles cancel.
Asymmetrical molecules
  • H₂O, NH₃, HCl and CHCl₃ are polar because their bond dipoles do not cancel.
Consequence
  • Polar molecules have permanent dipole–dipole forces and are attracted to a charged rod.

Part 4 of 5Describing Polarity in Answers

A clear sequence earns the marks in questions about polar molecules.

1
Compare the electronegativity values of the two bonded atoms to see which attracts the pair more strongly.
2
Mark δ+ on the less electronegative atom and δ− on the more electronegative atom of each bond.
3
Draw the shape of the molecule using electron pair repulsion theory to show the true arrangement of the bonds.
4
Check whether the bond dipoles are equal and opposite so that they cancel, as in tetrahedral CCl₄.
5
State that the molecule is polar only if an overall dipole remains, for example in H₂O or NH₃.

Part 5 of 5Exam Checklist & Tips

1.3.6 – Key Points for the Exam

  • Electronegativity The ability of an atom to attract the bonding pair of electrons in a covalent bond.
  • Polar bond A covalent bond in which the electrons are shared unequally.
  • Dipole A separation of partial positive and negative charge.
  • Pauling scale The commonly used scale of electronegativity values.

Common Mistakes

  • Saying a molecule with polar bonds is always polar; shape can cancel the dipoles.
  • Confusing electronegativity with electron affinity.
  • Stating that charge in a polar bond is a full charge, not a partial charge.
  • Describing a polar bond as ionic.

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