Electronegativity and bond polarityEdexcel A-Level Chemistry: Revision notes
Section 1
Electronegativity
Electronegativity is the ability of an atom to attract the bonding electrons in a covalent bond. It is measured on the Pauling scale; fluorine is the most electronegative element (4.0), followed by oxygen (3.5), nitrogen and chlorine (3.0).
Electronegativity increases across a period and decreases down a group, so the most electronegative elements are at the top right of the periodic table (excluding the noble gases). Typical values: H 2.1, C 2.5, N 3.0, O 3.5, F 4.0, Cl 3.0, Na 0.9.
Electronegativity is about an atom in a covalent bond, not the ability to attract any electron. Do not confuse it with electron affinity.
Section 2
Polar bonds
When two atoms of different electronegativity form a covalent bond, the bonding pair is pulled towards the more electronegative atom. That atom gains a partial negative charge (δ−) and the other a partial positive charge (δ+). The bond is a polar bond with a dipole.
- H–Cl: Hδ⁺–Clδ⁻ (difference 0.9)
- H–F: Hδ⁺–Fδ⁻ (difference 1.9, more polar)
- C–H: difference 0.4, almost non-polar
- Cl–Cl, O=O: identical atoms, difference zero, non-polar.
The larger the electronegativity difference, the more polar the bond.
Section 3
The ionic–covalent continuum
Pure covalent bonding (equal sharing) and pure ionic bonding (complete transfer) are two extremes. Real bonds lie on a continuum between them.
- Difference 0: pure covalent (Cl₂)
- Small difference: polar covalent (HCl, 0.9)
- Large difference: largely ionic (NaCl, 2.1; MgO, 2.3).
There is no sharp boundary: bonds with an intermediate difference have both covalent and ionic character. For example Al–Cl (1.5) is more covalent than Na–Cl (2.1), and aluminium chloride forms covalent molecules.
Always quote electronegativity differences from the data given. A difference of 0 is pure covalent; the larger it is, the more ionic.
Section 4
Polar bonds and polar molecules
A polar molecule has an overall dipole. A molecule can contain polar bonds and still be non-polar, if the bond dipoles are equal and cancel because of the symmetrical shape.
- Non-polar overall: CO₂ (linear), BF₃ (trigonal planar), CH₄ and CCl₄ (tetrahedral), BeCl₂ (linear), SF₆ (octahedral)
- Polar: H₂O (bent), NH₃ (pyramidal), CHCl₃ and CH₃Cl (tetrahedral but not symmetrical), HCl.
To predict polarity: (1) are the bonds polar? (2) what is the shape? (3) do the dipoles cancel? A simple test is a thin stream of liquid from a burette: polar liquids are deflected by a charged rod; non-polar liquids are not.
Do not say a molecule is non-polar because it has no polar bonds. CO₂ and CCl₄ have polar bonds; it is the symmetrical shape that cancels the dipoles.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Electronegativity and bond polarity
- A chemist compares the hydrogen halides. The Pauling electronegativity values are: H 2.1, F 4.0, Cl 3.0, Br 2.8 and I 2.5.Explain why the bond in hydrogen fluoride is polar.2 marks
- Some Pauling electronegativity values are: Na 0.9, Mg 1.2, Al 1.5 and Cl 3.0. A student compares the bonding in the chlorides NaCl, MgCl₂ and AlCl₃.Explain what is meant by saying that ionic and covalent bonding are the extremes of a continuum, using the bonding in aluminium chloride as an example.2 marks
- A teacher brings a charged plastic rod close to a thin stream of liquid running from a burette. The streams of water and trichloromethane, CHCl₃, are deflected towards the rod. The streams of tetrachloromethane, CCl₄, and hexane are not deflected. The Pauling electronegativity values are: H 2.1, C 2.5 and Cl 3.0.Explain why tetrachloromethane is not deflected even though it contains polar bonds.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).