The big idea: Ionic, covalent and metallic bonding are not three separate boxes — they are the three extremes of one continuous range. Most real substances bond somewhere in between.
We show this with the bonding triangle (the van Arkel–Ketelaar triangle). Its three corners are the pure bonding types, and any compound is placed inside using just two numbers worked out from electronegativity.
Metallic (bottom-left), covalent (bottom-right) and ionic (top) are the extremes — real compounds sit somewhere inside.
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The three corners: - Metallic (bottom-left): atoms of low electronegativity sharing a sea of delocalised electrons. - Covalent (bottom-right): atoms of high, similar electronegativity sharing electron pairs. - Ionic (top): a large difference in electronegativity, so electrons are transferred from one atom to the other.
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Electronegativity (χ) is how strongly an atom pulls a shared pair of electrons towards itself. Each element has a fixed value in the data booklet (e.g. Na 0.9, Cl 3.2). To place a compound made of two elements A and B, you work out two things from their χ values.
- average electronegativity (horizontal axis →)
- difference in electronegativity (vertical axis ↑)
- the electronegativities of the two elements
What each number controls: - Δχ (the difference) sets the height — a bigger Δχ means electrons are pulled to one atom, so the bonding is more ionic (further up). - χavg (the average) sets the left–right position — a low average → metallic; a high average with a small difference → covalent.
Δχ rises up the triangle (more ionic); χavg rises left → right (metallic → covalent).
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The method
- Look up the electronegativity (χ) of each element in the data booklet.
- Average them for the horizontal position: χavg = (χA + χB)/2.
- Subtract (largest − smallest) for the vertical position: Δχ = |χA − χB|.
- Plot the point: high Δχ → ionic (top); low Δχ + high χavg → covalent; low Δχ + low χavg → metallic.
Worked example — where does MgO sit?
Use χ(Mg) = 1.3 and χ(O) = 3.4 to find χavg and Δχ for magnesium oxide, MgO, and state the bonding type.
Solution
- Formula first — the average sets the horizontal position:
- The difference sets the vertical position:
- A large Δχ (2.1) puts the point high up the triangle, so the bonding is predominantly ionic — as expected for a metal + non-metal.
Final answer
χavg = 2.35, Δχ = 2.1 → high up the triangle → ionic bonding.
Same idea, three answers: - Na (χavg low, Δχ ≈ 0) → bottom-left → metallic. - Cl2 (χavg high, Δχ = 0) → bottom-right → covalent. - NaCl (Δχ large) → top → ionic.
The only thing that changed was the two electronegativity values.
How this is tested: S2.4.1 shows up two ways.
- Paper 1A (MCQ): given a compound's χavg and Δχ, identify which bonding type (or which is the alloy/ionic/covalent substance). - Paper 2: a deduce / determine part — calculate Δχ (and sometimes a % covalent character) from data-booklet electronegativities, then state the bonding type and where it sits on the triangle.
Markers want the calculation shown and the conclusion justified by the size of Δχ — not just a guess.
Don't rely on 'metal + non-metal = ionic': The rule of thumb fails for many compounds. BeCl2 is a metal + non-metal yet is largely covalent (small Δχ). The triangle uses the numbers, so it gives the right answer where the simple rule does not.
IB-style question — aluminium chloride (a)
Aluminium chloride, AlCl3, has χ(Al) = 1.6 and χ(Cl) = 3.2. (a) Determine χavg and Δχ, and deduce the dominant bonding type. [3]
How to score the marks
- Mark 1 — average (χavg): χavg = (1.6 + 3.2)/2 = 2.4.
- Mark 2 — difference (Δχ): Δχ = |1.6 − 3.2| = 1.6.
- Mark 3 — deduce: a Δχ of 1.6 is intermediate (not large), and χavg is high, so the point sits towards the covalent side of the triangle — AlCl3 is best described as polar covalent, not purely ionic.
Final answer
χavg = 2.4, Δχ = 1.6 → mid/lower-right of the triangle → polar covalent.
IB-style question — comparing two compounds (b)
(b) Potassium fluoride, KF, has Δχ = 3.2; silicon carbide, SiC, has Δχ = 0.6. Deduce which compound is more ionic, and explain your answer using the bonding triangle. [2]
How to score the marks
- Mark 1 — choice: KF is more ionic.
- Mark 2 — justify: KF has the larger Δχ (3.2 vs 0.6), so it sits higher up the triangle — electrons are essentially transferred. SiC's small Δχ keeps it near the covalent corner (electrons shared).
Final answer
KF — its much larger Δχ places it near the ionic apex, whereas SiC's small Δχ sits near the covalent corner.