A level Chemistry · Edexcel 9CH0 · Topic 2D
Electronegativity and polarity
Key points
- Electronegativity is the ability of an atom to attract the bonding electrons in a covalent bond. Fluorine is the most electronegative (Pauling scale 4.0).
- It increases across a period (more protons, similar shielding) and decreases down a group.
- A difference in electronegativity makes a polar bond with partial charges δ+ and δ−; a large difference tends towards ionic bonding.
- Bonding is a continuum: ionic compounds can have covalent character when a small, highly charged cation polarises a large anion.
- A molecule is polar only if its dipoles do not cancel: CO₂ and CCl₄ are non-polar because they are symmetrical; H₂O and CHCl₃ are polar.
- Test: a charged rod deflects a stream of a polar liquid but not a non-polar one.
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Electrons in a bond are not always shared equally. Electronegativity tells you which way the electrons are pulled, and combining bond polarity with molecular shape tells you whether a whole molecule is polar.
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