Bonding is not a simple classification of "ionic OR covalent OR metallic." In reality, most bonds exhibit a mixture of character. The van Arkel-Ketelaar bonding triangle maps bond type as a continuum using two electronegativity parameters.
| Triangle Vertex | ΔEN | Average EN | Bonding Character |
|---|---|---|---|
| Top | High | Medium | Ionic (electron transfer) - e.g. NaCl |
| Bottom-right | Low/zero | High | Covalent (electron sharing) - e.g. Cl₂ |
| Bottom-left | Low/zero | Low | Metallic (electron delocalisation) - e.g. Na |
Period 3 Oxides - The Bonding Transition
Period 3 Chlorides & Bonding Continuum
Moving across Period 3 from left to right:
- Na₂O, MgO → Giant ionic: high mp, conduct when molten, dissolve to give basic/amphoteric solutions
- Al₂O₃ → Transitional: very high mp, significant covalent character due to small, highly charged Al³⁺
- SiO₂ → Giant covalent: extremely high mp, insoluble, insulator
- P₄O₁₀, SO₃, Cl₂O → Simple molecular: low mp, dissolve in water to form acidic solutions
Bonding as a Discrete Classification Trap
The bonding triangle is a model, not absolute reality. Some compounds sit in "grey areas" - for example, AlCl₃ is classified as ionic but behaves as a covalent dimer when vaporised. Always discuss bonding as a continuum.
Bonding Triangle Questions on Paper 2
To locate a compound on the triangle, you must calculate two values: (1) the average electronegativity of the bonded atoms (x-axis) and (2) the difference in electronegativity (ΔEN) between them (y-axis). Show both calculations explicitly - missing either one loses marks.
Study this topic on the go
Get active recall flashcards, notes, and topic quizzes in ChemEasy, or build your revision schedule with ChemPlan IB.