Bonding Triangle Foundation
The position of a compound in the bonding triangle is determined by the relative contributions of the three bonding types to the overall bond.
Placing a Compound on the Triangle
To locate a binary compound on the bonding triangle, you need the electronegativity values (χ) of the two elements from the IB Data Booklet. Calculate two quantities:
X-Axis: Average Electronegativity (Σχ / 2)
\[\text{Average } \chi = \frac{\chi_A + \chi_B}{2}\]
A higher average χ shifts the point to the right, toward the covalent corner.
Y-Axis: Electronegativity Difference (Δχ)
\[\Delta\chi = |\chi_A - \chi_B|\]
A higher Δχ pushes the point upward, toward the ionic corner.
Bonding Triangle with Example Compounds
Interpreting the Three Regions
| Region | Average χ | Δχ | Bonding type | Typical properties |
|---|---|---|---|---|
| Bottom-left | Low | Low (≈ 0) | Metallic | Conducts as solid, malleable, lustrous |
| Bottom-right | High | Low (≈ 0) | Covalent | Low melting point (molecular) or very high (network) |
| Top | Medium | High | Ionic | High MP, conducts when molten/dissolved, brittle |
| Middle | Medium-High | Medium | Polar covalent | Properties intermediate between ionic and covalent |
Worked Example
Plotting Magnesium Oxide (MgO)
From the Data Booklet: \(\chi(\text{Mg}) = 1.31\), \(\chi(\text{O}) = 3.44\)
- Average \(\chi = (1.31 + 3.44) / 2 = \mathbf{2.38}\)
- \(\Delta\chi = |1.31 - 3.44| = \mathbf{2.13}\)
- ∴ MgO is plotted in the ionic region (high \(\Delta\chi\), medium average \(\chi\)).
- Prediction: High melting point, conducts when molten, brittle, soluble in water.
Plotting Hydrogen Fluoride (HF)
From the Data Booklet: \(\chi(\text{H}) = 2.20\), \(\chi(\text{F}) = 3.98\)
- Average \(\chi = (2.20 + 3.98) / 2 = \mathbf{3.09}\)
- \(\Delta\chi = |2.20 - 3.98| = \mathbf{1.78}\)
- ∴ HF sits between the ionic and covalent regions (significant polar covalent character).
- Prediction: Hydrogen bonding, relatively high boiling point for its size, exists as a liquid at room temperature.
van Arkel-Ketelaar Applies to Binary Compounds Only
The IB syllabus states that only binary compounds (compounds of exactly two elements) need to be considered for bonding triangle questions. You do not need to calculate percentage ionic character.
Predicting Properties from Position
The real power of the bonding triangle is being able to predict the physical properties of a substance from where it sits on the diagram.
Reading Coordinates from Data Booklet Section 8
When asked to predict properties from a triangle position:
- Identify the dominant bonding type from the region (ionic, covalent, or metallic)
- Apply the properties you learned in S2.1 (ionic), S2.2 (covalent), or S2.3 (metallic)
- Acknowledge any mixed character if the compound falls near a boundary
Real Materials Lie Along Gradients
No compound has 100% pure ionic, covalent, or metallic bonding. Even NaCl has some covalent character. The triangle shows a spectrum of bonding. Compounds near the centre or along the edges display intermediate properties. For example, AlCl₃ is classified as ionic but behaves as a covalent dimer when vaporised.
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