2.4.1Structure 2 — Models of bonding and structure
Bonding Models — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 5 of 6 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 6The Three Bonding Types
No single model describes every substance; the models overlap.
Ionic
- Ionic Electrons are transferred from metal to non-metal, producing ions held by electrostatic attraction in a lattice.
Covalent
- Covalent Electrons are shared between non-metal atoms in a bond that attracts both nuclei.
Metallic
- Metallic Cations are held in a lattice by a sea of delocalised electrons shared by all atoms.
Real substances
- Real substances Most real substances show features of more than one type, so the types are idealised models.
Section 2 of 6The Bonding Triangle
The Bonding Triangle
Two axes
- Two axes The vertical axis shows the electronegativity difference, ΔEN, and the horizontal axis shows the average electronegativity.
Ionic corner
- Ionic corner Large ΔEN with a low average electronegativity suggests an ionic compound, such as CsF.
Metallic corner
- Metallic corner Small ΔEN with a low average electronegativity suggests metallic bonding, as in sodium.
Covalent corner
- Covalent corner Small ΔEN with a high average electronegativity suggests covalent bonding, as in Cl₂.
Section 3 of 6Using the Triangle
1
Look up the electronegativity of each element in the data booklet.
2
Calculate the difference ΔEN between the two elements.
3
Calculate the average electronegativity by adding the two values and dividing by two.
4
Plot the point on the triangle and see which corner it lies closest to.
5
State the dominant bonding type and mention any mixed character if the point lies between corners.
| Compound | ΔEN | Average EN | Predicted type |
|---|---|---|---|
| NaCl | 2.3 | 2.05 | ionic |
| HCl | 1.0 | 2.7 | polar covalent |
| Cl₂ | 0 | 3.2 | covalent |
Section 4 of 6Intermediate Bonding
Polar covalent
- Polar covalent Bonds between different non-metals, such as H–Cl, lie between covalent and ionic.
Ionic with covalent character
- Ionic with covalent character Small, highly charged cations can distort anions, so compounds like Al₂O₃ show some covalent character.
Semi-metals
- Semi-metals Elements such as silicon sit between metallic and covalent regions of the triangle.
Data limits
- Data limits Electronegativity values are approximate, so the triangle gives a trend and not a rigid rule.
Section 5 of 6Worked Example
Question Use electronegativity values (Na 0.9, Cl 3.2) to predict the bonding in sodium chloride.
- ΔEN = 3.2 − 0.9 = 2.3.
- Average EN = (3.2 + 0.9) ÷ 2 = 2.05.
- A large ΔEN means the point lies towards the ionic corner of the triangle.
Answer Sodium chloride is predominantly ionic.
Section 6 of 6Exam Checklist & Tips
2.4.1 – Key Points for the Exam
- Bonding triangle A diagram that places compounds between ionic, covalent and metallic bonding using electronegativity.
- Continuum A range in which one type of bonding gradually changes into another.
- ΔEN The difference in electronegativity between two bonded atoms.
- Polar covalent bond A covalent bond with unequal electron sharing.
Common Mistakes
- Treating bonding types as completely separate categories.
- Using the sum of electronegativities rather than the average.
- Using ΔEN alone to classify a compound without considering metals or non-metals.
- Forgetting that the triangle is a model and not an exact boundary.
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