IB DP Chemistry - S2.4.2 Position in the bonding triangle- Study Notes - New Syllabus - 2026, 2027 & 2028
IB DP Chemistry – S2.4.2 Position in the bonding triangle- Study Notes – New Syllabus
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Structure 2.4.2 — The Bonding Triangle: Position and Prediction
Structure 2.4.2 — The Bonding Triangle: Position and Prediction
Bonding Triangle
The bonding triangle is a conceptual model used to visualize the bonding character of a substance based on the three main types of bonding:
- Ionic: complete transfer of electrons from metal to non-metal (e.g. NaCl)
- Covalent: sharing of electrons between non-metals (e.g. Cl2, H2O)
- Metallic: a lattice of metal cations in a sea of delocalized electrons (e.g. Cu, Al)
Most real-world compounds do not fall at the corners, but somewhere within the triangle, indicating a mixture of bonding characters.
Role of Electronegativity in Bond Type
Electronegativity \( (\chi) \) is a measure of how strongly an atom attracts electrons in a bond. The difference in electronegativity between bonded atoms \( (\Delta \chi) \) determines the bond’s character:
- \( \Delta \chi > 1.8 \) → Bond is mainly ionic
- \( 0.4 < \Delta \chi < 1.8 \) → Bond is polar covalent
- \( \Delta \chi \approx 0 \) → Bond is nonpolar covalent
- Metals bonding with metals → metallic bonding
Sample Electronegativity Values (Pauling Scale):
- Na: 0.9
- Mg: 1.2
- Al: 1.5
- C: 2.5
- H: 2.1
- Cl: 3.0
- O: 3.5
How to Place a Compound in the Triangle
- Find the electronegativities of both atoms
- Calculate \( \Delta \chi = | \chi_{\text{A}} – \chi_{\text{B}} | \)
- Use the value to classify the bond type
- Predict bonding character and likely physical properties
Predicting Properties Based on Bond Type
| Bond Type | Electrical Conductivity | Melting/Boiling Point | Solubility |
|---|---|---|---|
| Ionic | Conducts when molten or in aqueous solution | High | Soluble in water |
| Simple Covalent | Non-conductive | Low | Soluble in nonpolar solvents |
| Giant Covalent | Usually non-conductive (except graphite) | Very high | Insoluble |
| Metallic | Conducts in solid and molten states | Moderate to high | Insoluble |
Binary Compound Examples with Varying Bonding Character
Compound: NaCl
Electronegativity Difference: \( \Delta \chi = 3.0 – 0.9 = 2.1 \)
Bonding Character: Mostly Ionic
- High melting point
- Soluble in water
- Conducts when molten/aqueous
Compound: MgO
Electronegativity Difference: \( \Delta \chi = 3.5 – 1.2 = 2.3 \)
Bonding Character: Highly Ionic
- Very high melting point
- Insoluble in organic solvents
- Good conductor when molten
Compound: HCl
Electronegativity Difference: \( \Delta \chi = 3.0 – 2.1 = 0.9 \)
Bonding Character: Polar Covalent
- Low boiling point
- Soluble in water
- Conducts as ions in solution
Compound: Cl2
Electronegativity Difference: \( \Delta \chi = 0 \)
Bonding Character: Pure Covalent
- Very low melting/boiling point
- Insoluble in water
- Non-conductive
Compound: AlCl3
Electronegativity Difference: \( \Delta \chi = 3.0 – 1.5 = 1.5 \)
Bonding Character: Covalent with Ionic Character
- Sublimes at low temperature
- Soluble in organic solvents
- Forms dimers in gas phase
Example
Using the electronegativity values below, determine the predominant bonding type in aluminum oxide (Al2O3).
Al = 1.5, O = 3.5
▶️Answer/Explanation
Step 1: Calculate the electronegativity difference:
\( \Delta \chi = 3.5 – 1.5 = 2.0 \)
Step 2: Since \( \Delta \chi > 1.8 \), this suggests mostly ionic bonding.
However, because Al3+ is small and highly charged, it can polarize the oxide ion, introducing covalent character.
Final conclusion: Al2O3 lies close to the ionic corner but with some covalent character.
Properties: high melting point, conducts when molten, insoluble in water.
Example
Predict the bonding type and properties of hydrogen fluoride (HF).
Electronegativity: H = 2.1, F = 4.0
▶️Answer/Explanation
Step 1: \( \Delta \chi = 4.0 – 2.1 = 1.9 \)
Step 2: This value is close to the ionic threshold but still in the polar covalent range.
HF forms a strong polar covalent bond, with strong hydrogen bonding between molecules.
Properties: High boiling point (for a small molecule), soluble in water, non-conductive as a pure liquid.
Example
Carbon tetrachloride (CCl4) is a liquid at room temperature. Use bonding models to explain its structure and physical properties.
Electronegativity: C = 2.5, Cl = 3.0
▶️Answer/Explanation
Step 1: \( \Delta \chi = 3.0 – 2.5 = 0.5 \) → polar covalent bonds.
However, the molecule is tetrahedral and symmetrical, so dipoles cancel → non-polar molecule.
Bonding: Covalent, with weak London forces between molecules.
Properties: Low melting/boiling point, insoluble in water, non-conductive.