Electronegativity and bond polarityEdexcel International A Level Chemistry: Revision notes
Section 1
Electronegativity
Electronegativity is the ability of an atom to attract the bonding pair of electrons in a covalent bond. It is measured on the Pauling scale, on which fluorine is highest (3.98).
Two atoms with the same electronegativity share the pair equally (a non-polar bond, as in Cl–Cl). If the atoms differ, the pair is pulled towards the more electronegative atom.
Electronegativity is the pull of an atom on the bonding pair in a bond, not its pull on all its own electrons.
Section 2
Polar bonds
In a polar bond the electrons are shared unequally. The more electronegative atom has a partial negative charge, δ−, and the other atom a partial positive charge, δ+. The bond has a dipole.
The larger the electronegativity difference, the more polar the bond. For example, C–F (difference 1.43) is more polar than C–Cl (0.61), and C–H (0.35) is almost non-polar.
To state the direction, name the atom that is δ−: the one with the higher electronegativity.
Show the working: write the two electronegativity values, subtract, and then name the atom that is δ−.
Section 3
The ionic–covalent continuum
Ionic and covalent bonding are the two extremes of a continuum. If the electronegativity difference is zero the bond is non-polar covalent. As the difference increases the bond becomes more polar. If the difference is large enough, the more electronegative atom removes the electron completely and the bonding is ionic.
- NaCl: difference 2.23, ionic
- HCl: difference 0.96, polar covalent
- Cl₂: difference 0, non-polar covalent
Compounds such as SiCl₄ (1.26) lie between the extremes.
Section 4
Polar molecules
A polar molecule has a net dipole. This needs polar bonds and a shape in which the dipoles do not cancel.
- CO₂ (linear), CCl₄ (tetrahedral) and BF₃ (trigonal planar) have polar bonds but are non-polar, because the dipoles are arranged symmetrically and cancel.
- H₂O (bent), NH₃ (pyramidal) and CH₃Cl (tetrahedral but with one different atom) are polar, because the dipoles do not cancel.
To predict polarity, work out the polarity of each bond, then the shape, then ask whether the dipoles cancel.
A molecule with polar bonds is not automatically polar. Always check whether the shape makes the dipoles cancel.
Must know
- Electronegativity: ability of an atom to attract the bonding pair in a covalent bond
- Larger difference gives a more polar bond; the more electronegative atom is δ−
- Ionic and covalent bonding are extremes of a continuum
- A polar molecule needs polar bonds and a shape where the dipoles do not cancel
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Electronegativity and bond polarity
- Pauling electronegativity values: H 2.20, C 2.55, Cl 3.16, F 3.98 and Na 0.93.Explain, using the data, why sodium chloride is ionic whereas hydrogen chloride is polar covalent.2 marks
- Pauling electronegativity values: H 2.20, C 2.55, O 3.44 and Cl 3.16. Carbon dioxide is a linear molecule (O=C=O), water is a bent (V-shaped) molecule, and chloromethane, CH₃Cl, methane, CH₄, and tetrachloromethane, CCl₄, are all tetrahedral.Explain why water is a polar molecule.2 marks
- Pauling electronegativity values: H 2.20, F 3.98, Cl 3.16, I 2.66, Na 0.93 and Si 1.90.Calculate the electronegativity difference in H–F, H–Cl and H–I. State which bond is the most polar and the direction of the dipole in that bond.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).