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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

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Part 1 of 6The Three Bonding Types

2.4.1 Bonding Models

No single model describes every substance; the models overlap.

Ionic
  • Electrons are transferred from metal to non-metal, producing ions held by electrostatic attraction in a lattice.
Covalent
  • Electrons are shared between non-metal atoms in a bond that attracts both nuclei.
Metallic
  • Cations are held in a lattice by a sea of delocalised electrons shared by all atoms.
Real substances
  • Most real substances show features of more than one type, so the types are idealised models.

Part 2 of 6The Bonding Triangle

The Bonding Triangle
Two axes
  • The vertical axis shows the electronegativity difference, ΔEN, and the horizontal axis shows the average electronegativity.
Ionic corner
  • Large ΔEN with a low average electronegativity suggests an ionic compound, such as CsF.
Metallic corner
  • Small ΔEN with a low average electronegativity suggests metallic bonding, as in sodium.
Covalent corner
  • Small ΔEN with a high average electronegativity suggests covalent bonding, as in Cl₂.

Part 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.
CompoundΔENAverage ENPredicted type
NaCl2.32.05ionic
HCl1.02.7polar covalent
Cl₂03.2covalent

Part 4 of 6Intermediate Bonding

Polar covalent
  • Bonds between different non-metals, such as H–Cl, lie between covalent and ionic.
Ionic with covalent character
  • Small, highly charged cations can distort anions, so compounds like Al₂O₃ show some covalent character.
Semi-metals
  • Elements such as silicon sit between metallic and covalent regions of the triangle.
Data limits
  • Electronegativity values are approximate, so the triangle gives a trend and not a rigid rule.

Part 5 of 6Worked Example

Question Use electronegativity values (Na 0.9, Cl 3.2) to predict the bonding in sodium chloride.

  1. ΔEN = 3.2 − 0.9 = 2.3.
  2. Average EN = (3.2 + 0.9) ÷ 2 = 2.05.
  3. A large ΔEN means the point lies towards the ionic corner of the triangle.

Answer Sodium chloride is predominantly ionic.

Part 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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