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1.5.1Physical Chemistry

Electronegativity — Edexcel AS Chemistry Revision Notes & Topical Questions

5 min read 4 parts Lesson 18 of 42

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Part 1 of 4Definition and Scale

1.5.1 Electronegativity

Differences in electronegativity decide how the electron pair in a bond is shared.

Definition
  • Electronegativity is the ability of an atom in a covalent bond to attract the bonding pair of electrons towards itself.
Pauling scale
  • Values run from about 0.7 to 4.0, with fluorine the most electronegative element at 4.0.
Common values
  • H 2.1, C 2.5, N 3.0, Cl 3.0, O 3.5 and F 4.0, which are useful for working out polarity.
Factors
  • Electronegativity increases with nuclear charge and decreases with distance and shielding.
Polar bond example
  • In H–Cl the chlorine atom has the greater electronegativity of 3.0 against 2.1, so chlorine carries δ⁻ and hydrogen carries δ⁺.

Part 2 of 4Trends

Trends
Across a period
  • Electronegativity increases, because nuclear charge rises while the bonding electrons stay in the same shell.
Down a group
  • Electronegativity decreases, because the bonding pair is further from the nucleus and more shielded.
Most extreme
  • Fluorine, in the top right of the Periodic Table, is highest, and caesium and francium are lowest.
Noble gases
  • These usually have no electronegativity values because they rarely form bonds.
Link to bonding
  • Ionic character increases as the difference in electronegativity between two atoms increases,
    • so bonding forms a continuum from covalent to ionic.

Part 3 of 4Bond Polarity

1
If two atoms have the same electronegativity, as in Cl₂, the bonding pair is shared equally and the bond is non-polar.
2
If the atoms differ, the more electronegative atom has the larger share of the electron density, giving it a partial negative charge, δ⁻.
3
The other atom becomes partially positive, δ⁺, and the bond is a polar covalent bond with a permanent dipole.
4
A very large difference may transfer the electron completely, giving ionic bonding.
5
A molecule is polar overall only if its dipoles do not cancel: CO₂ and CCl₄ are symmetrical and non-polar, but H₂O and NH₃ are polar.

Part 4 of 4Exam Checklist & Tips

1.5.1 – 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 with unequal sharing of electrons.
  • Dipole A separation of partial positive and partial negative charge, shown as δ⁺ and δ⁻.
  • Pauling scale The scale used to give electronegativity values, with fluorine at 4.0.

Common Mistakes

  • Saying that every molecule with polar bonds is polar; symmetrical molecules can have no overall dipole.
  • Describing electronegativity as the ability to attract all electrons rather than the bonding pair.
  • Thinking the noble gases are the most electronegative group.
  • Forgetting that δ⁺ and δ⁻ are partial charges, not full ionic charges.

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