2.4.1Structure 2 — Models of bonding and structure
Bonding Models — IB Chemistry HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 5 of 6 Unit quiz
Part 1 of 5The Three Models
No single model describes all bonding, so chemists choose the model that best fits a substance.
Ionic model
- Electrostatic attraction between oppositely charged ions in a lattice.
Covalent model
- Attraction between shared electron pairs and the nuclei of the bonded atoms.
Metallic model
- Attraction between cations and a sea of delocalised electrons.
Models
- Each model simplifies reality, and real bonding often lies between the models.
Part 2 of 5Electronegativity and Bond Type
Electronegativity and Bond Type
Large difference
- Metal and non-metal combinations with large electronegativity differences are ionic.
Small difference, high values
- Two non-metals share electrons and form covalent bonds, polar if the difference is moderate.
Low values
- Metals with low electronegativity form metallic bonds.
Continuum
- The bond types merge gradually, with no strict dividing lines.
Part 3 of 5The Bonding Triangle
1
Plot the average electronegativity of the two atoms on the horizontal axis.
2
Plot the difference in electronegativity on the vertical axis.
3
Compounds with a large difference lie near the top corner, which represents ionic bonding.
4
Compounds with a small difference and a high average lie near the covalent corner.
5
Compounds with a small difference and a low average lie near the metallic corner.
| Substance | Average EN | EN difference | Closest to |
|---|---|---|---|
| Cl₂ | 3.2 | 0.0 | Covalent |
| Na | 0.9 | 0.0 | Metallic |
| HCl | 2.7 | 1.0 | Polar covalent |
| MgO | 2.35 | 2.1 | Ionic |
Part 4 of 5Applying the Models
Predicting properties
- Find the bonding type first, and then use it to predict melting point and conductivity.
Intermediate bonding
- Compounds in the middle of the triangle, such as AlCl₃, show both ionic and covalent character.
Limitations
- Models do not explain every property, so evidence from experiments is needed.
Part 5 of 5Exam Checklist & Tips
2.4.1 – Key Points for the Exam
- Bonding triangle A diagram placing bonding types according to electronegativity values.
- Electronegativity difference The difference between the electronegativity values of two bonded atoms.
- Ionic character The extent to which a bond resembles complete electron transfer.
- Model A simplified representation used to explain and predict observations.
Common Mistakes
- Treating the three bond types as completely separate.
- Using atomic mass instead of electronegativity to classify bonds.
- Forgetting that metals also bond covalently in some compounds.
- Mixing up the axes of the bonding triangle.
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