All subjects · Chemistry · WCH11 (Unit 1)

Electronegativity, polarity and polarisation

Few bonds are purely ionic or purely covalent. Electronegativity and polarisation explain where a bond sits between the two.

Electronegativity

Electronegativity is the ability of an atom to attract the bonding pair of electrons in a covalent bond. It increases across a period and up a group; fluorine is the most electronegative element (Pauling scale 4.0). It increases with greater nuclear charge and smaller atomic radius.

Polar bonds and polar molecules

If the two atoms differ in electronegativity, the bond is polar: δ+ on the less electronegative atom, δ- on the more. A molecule is polar only if the bond dipoles do not cancel. CO2 and CCl4 have polar bonds but are non-polar because they are symmetrical; H2O, NH3 and CHCl3 are polar.

The ionic-covalent continuum

A larger electronegativity difference means more ionic character. Pure covalent bonding is between identical atoms (Cl2); bonds between very different atoms (NaF) are almost fully ionic; most bonds lie in between.

Polarisation

A small, highly charged cation has high polarising power: it distorts the electron cloud of a nearby anion. Large anions are more polarisable. Strong polarisation pulls electron density between the ions, giving the bond some covalent character. So AlI3 (small 3+ cation, large anion) is much more covalent than NaF.

Worked example

Explain why CCl4 is non-polar even though the C-Cl bond is polar.

  1. Cl is more electronegative than C, so each C-Cl bond is polar.
  2. The molecule is tetrahedral and symmetrical.
  3. The four bond dipoles cancel.

Answer: No overall dipole, so CCl4 is non-polar.

Key idea: Electronegativity: attraction for a bonding pair. Polar bonds in a symmetrical molecule can cancel. Small, highly charged cations polarise large anions, adding covalent character.

Check yourself

  1. Which element is the most electronegative?
    • Fluorine
    • Chlorine
    • Oxygen
    • Caesium
    Show answer

    Fluorine. F has a high nuclear charge for its very small size.

  2. Which molecule is polar?
    • CO2
    • CCl4
    • NH3
    • BF3
    Show answer

    NH3. NH3 is pyramidal, so its dipoles do not cancel. The others are symmetrical.

  3. Which compound has the most covalent character?
    • AlI3
    • NaF
    • KCl
    • NaCl
    Show answer

    AlI3. Al3+ is small and highly charged; I- is large and easily polarised.

  4. In an H-Cl bond, which atom carries the δ- charge?
    • H
    • Neither, the bond is non-polar
    • It changes back and forth
    • Cl
    Show answer

    Cl. Chlorine is more electronegative, so it draws the bonding pair towards itself.

  5. Which cation has the greatest polarising power?
    • Na+
    • Mg2+
    • K+
    • Ba2+
    Show answer

    Mg2+. Mg2+ combines a 2+ charge with a small radius, giving the highest charge density.

  6. How does electronegativity change across Period 3 from Na to Cl?
    • It decreases
    • It stays the same
    • It increases then decreases
    • It increases
    Show answer

    It increases. Nuclear charge increases while atomic radius decreases.

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Last updated 7 November 2026. Spotted a mistake? Tell us.