IB Chemistry SLTopic 2 — From Bonding Models to MaterialsPaper 1 & 2Core idea~10 min read
Comparing Bonding Models
Ionic, covalent and metallic are models — simplified pictures that let us predict how a substance will behave. They work brilliantly most of the time. But real bonding doesn’t come in three tidy boxes, and the bonding triangle is how we deal with that.
📚 What you need to know
The three bonding models are idealised. Most real substances sit somewhere between them.
The bonding triangle places substances on a continuum, with ionic at the apex, metallic bottom left and covalent bottom right.
χ is the Greek letter chi, said “ky” to rhyme with “sky”.
The triangle is in Section 17 of the data booklet; electronegativities are in Section 9.
Where the simple rule breaks down
Up to now the rule has been easy: metal + non-metal means ionic, two non-metals means covalent, two metals means metallic. Aluminium chloride shows why that isn’t the whole story.
The rule predicts a giant ionic lattice. The real melting point says otherwise.
The problem is that Al3+ is small and carries a 3+ charge. It pulls so hard on the electron cloud of each chloride ion that it drags electron density back towards itself. The result is a bond that is much closer to polar covalent than to a clean transfer of electrons — which is exactly what the low melting point tells us.
So instead of asking “is it ionic or covalent?”, a better question is “how ionic is it?”
Bonding as a continuum
Picture the three models as the three corners of a triangle. A substance sitting exactly at a corner is a perfect example of that model. Everything else sits somewhere inside, with a mixture of character.
The three models are the corners. Real substances are scattered across the whole area.
You met a simpler version of this back in Bond Polarity — the sliding scale from nonpolar covalent through polar covalent to ionic. The bonding triangle is the same idea with metallic bonding added as a third corner.
The two axes
To put a substance on the triangle you need two numbers, and both come from electronegativity values:
This tells you how strongly the atoms hold electrons in general. A low average means you are dealing with metals, pushing you left. A high average means non-metals, pushing you right.
A simplified version of the bonding triangle. The exam version, with exact region boundaries, is in Section 17 of the data booklet.
Reading the triangle
Once you know where a substance sits, the region tells you the bonding:
Elements have Δχ = 0 (both atoms identical), so they always sit on the x-axis. Where along it depends only on their electronegativity.
Metals have low electronegativity, so they sit at the bottom left — the metallic corner.
Non-metal elements have high electronegativity, so they sit at the bottom right — the covalent corner.
Ionic compounds have a large Δχ and sit near the apex.
Polar covalent compounds sit in the middle, between ionic and covalent.
WORKED EXAMPLE
Sodium has an electronegativity of 0.9 and chlorine 3.2. Calculate the coordinates of sodium chloride on the bonding triangle.
x-axis — average electronegativityχ = (3.2 + 0.9) ÷ 2= 2.05y-axis — electronegativity differenceΔχ = 3.2 − 0.9= 2.3Plot at (2.05, 2.3) — high up, near the apex, so strongly ionic.
💡 Exam tip
Always subtract the smaller electronegativity from the larger one, so Δχ is never negative.
You do not need to memorise the triangle or the electronegativity values — both are in the data booklet. What you need is the ability to calculate the two numbers and interpret the position.
Write the coordinates in the order (average, difference) — x first, as always.
For an alloy, treat it like a compound: use the electronegativities of the two metals.
⚠️ Common mix-up
Don’t swap the axes. Difference goes up the side, average goes along the bottom.
An element is not automatically at a corner. Its Δχ is zero so it sits on the x-axis, but its position along that axis depends on its electronegativity.
“Metal + non-metal = ionic” is a starting point, not a law. AlCl3 is the standard counter-example.
The triangle describes bonding character, not structure. You still need melting point and conductivity data to identify a giant lattice.
Up next: Bonding and Physical Properties — plotting real substances on the triangle and using their position to predict how they behave.
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