Worked example 1
Classify the bonds in Cl₂, HCl and NaCl using EN values Cl 3.16, H 2.20, Na 0.93.
Try it first: Compute each difference before classifying anything.
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Types of Solids and IMFSTypes of elementsContent level: Core 18 min
Bond Type and Electronegativity Difference
What you'll be able to do: Use electronegativity difference to classify a bond as nonpolar covalent, polar covalent or ionic.
Best after: Metals, Nonmetals and Metalloids, Atomic Radius, Ionization Energy and Electronegativity
Introduction
Bonding is a spectrum, not three separate boxes. Electronegativity difference tells you where on that spectrum a given bond sits.
These are recommended, not required. You can start this lesson at any time.
Learning objectives
- Calculate the electronegativity difference for a bond
- Classify bonds as nonpolar covalent, polar covalent or ionic
- Assign partial charges and a bond dipole direction
- Explain why bond type is a continuum
Lesson
Electronegativity difference sets the bond
Take the absolute difference between the two electronegativity values. A common classroom scheme: below about 0.4 is nonpolar covalent, 0.4 to about 1.7 is polar covalent, and above about 1.7 behaves as ionic. The cutoffs are conventions, not laws of nature.
deltaEN = |ENA - ENB|
Partial charges and the bond dipole
In a polar covalent bond the more electronegative atom carries a partial negative charge and the other a partial positive charge. The bond dipole arrow points from the partial positive atom toward the partial negative atom.
A partial charge is a fraction of an electron, not a full transfer.
Why the cutoffs are soft
HF has a difference of 1.9 yet is a molecular gas, while some metal halides with smaller differences behave ionically. Use the difference as a ranking tool and combine it with element type before declaring a compound ionic.
Rank bonds by polarity confidently; classify borderline cases with element type as well.
Polar bond is not the same as polar molecule
CO₂ has two strongly polar bonds, but they point in opposite directions and cancel, so the molecule is nonpolar overall. Molecular polarity needs geometry, which is covered in the bonding unit.
Separate bond polarity from molecular polarity
An electronegativity difference predicts an individual bond's unequal sharing, not the net dipole of the whole molecule. Bond dipoles are vectors: symmetric molecular geometry can cancel them. Therefore a molecule with polar bonds can be nonpolar overall. Use a Lewis structure and shape after identifying bond polarities.
First ask whether each bond is polar; then ask whether the dipoles cancel.
Key ideas
Definition
Electronegativity
A measure of how strongly an atom attracts the shared electrons in a bond.
Rule
Classification bands
Below 0.4 nonpolar covalent, 0.4 to 1.7 polar covalent, above 1.7 ionic.
Rule
Dipole direction
The arrow points toward the more electronegative atom.
Key concept
Geometry matters
Polar bonds can cancel, so a molecule with polar bonds may still be nonpolar.
Equation
Electronegativity difference
deltaEN = |ENA - ENB|
- deltaEN = electronegativity difference for the bond
- ENA = electronegativity of the first atom
- ENB = electronegativity of the second atom
Worked examples
Worked example 2
Rank the bonds C-H, O-H and N-H by polarity. EN values: H 2.20, C 2.55, N 3.04, O 3.44.
Try it first: All three share hydrogen, so only the partner matters.
0 of 3 steps revealed.
Worked example 3
Why is CO₂ nonpolar overall while H₂O is polar, although each contains polar bonds?
Try it first: Sketch the orientation of the bonds in each molecule.
0 of 3 steps revealed.
Common mistakes
Treating 1.7 as an exact boundary.
Why it's wrong: It is a convention with well known exceptions such as HF.
Check instead: Use the difference to rank polarity and check element types for borderline cases.
Saying a molecule with polar bonds must be polar.
Why it's wrong: Symmetric geometries cancel bond dipoles, as in CO₂ and CCl₄.
Check instead: Check the shape before assigning molecular polarity.
Drawing the dipole arrow toward the less electronegative atom.
Why it's wrong: Electron density shifts toward the stronger attractor.
Check instead: Point the arrow at the more electronegative atom.
After the lesson · Question bank
Independent practice
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What you should now know
The difference in electronegativity between two bonded atoms sets how unevenly the shared electrons sit. Small differences give nonpolar covalent bonds, intermediate differences give polar covalent bonds with partial charges, and large differences give essentially ionic bonding.
- Electronegativity difference places a bond on the ionic to covalent spectrum
- Partial charges appear on polar covalent bonds
- Cutoff values are conventions, not hard rules
- Polar bonds can cancel in symmetric molecules
Sources and further reading
- Chemistry 2e, Section 6.5: Periodic Variations in Element PropertiesFlowers, Theopold, Langley, Robinson · OpenStax, Rice University · Chapter 6.5View source
Access for free at openstax.org License