IB Chemistry SLTopic 2 — From Models to MaterialsPaper 1 & 2Core idea~11 min read
Comparing Bonding Models
Ionic, covalent, metallic. Three tidy boxes, and most substances drop neatly into one of them. Then you meet aluminium chloride — a metal joined to a non-metal that melts at a temperature you could reach in a school oven. This page is about what the three models are for, where they run out, and how one simple triangle fills the gap.
📘 What you need to know
The three bonding models are simplifications. Each one describes an extreme, not the whole truth.
Ionic = electrons handed over. Covalent = electrons shared between two atoms. Metallic = electrons let loose over many cations.
Real bonding sits on a continuum between the three, and most substances are somewhere in the middle.
Two numbers fix a substance’s position: the difference in electronegativity (Δχ) and the average electronegativity.
Δχ tells you how unevenly the electrons are shared. The average tells you how tightly the atoms hold electrons at all.
The bonding triangle plots those two numbers against each other. It is in Section 17 of the data booklet; electronegativity values are in Section 9.
You never have to memorise the triangle. You have to read it and say what a position means.
The three models in one place
A model is a picture you use to make a prediction. Each bonding model starts with a different assumption about what happens to the outer electrons, and each one leads to a different set of properties. Learn the chain in that order — picture, then prediction — and you can rebuild the properties from scratch instead of memorising lists.
Notice that the only real difference is what the outer electrons are doing. Everything else — melting point, conductivity, brittleness — follows from that one choice.
Model
What the model assumes
Typical example
What it predicts
Ionic
Outer electrons are transferred from metal to non-metal, giving oppositely charged ions in a giant lattice
Magnesium oxide, MgO
Very high melting point, brittle, conducts only when molten or dissolved
Covalent
A pair of electrons is shared between two non-metal atoms
Chlorine, Cl2
Low melting point if simple molecules, no conductivity, often soluble in non-polar solvents
Metallic
Outer electrons are delocalised across a lattice of cations
Magnesium, Mg
Conducts as a solid, malleable, usually high melting point
If you can only remember one line from this page, make it this one: the model is not the substance. The model is a prediction machine, and every prediction machine has a range where it works.
Where the models break down
Take aluminium chloride. Aluminium is a metal, chlorine is a non-metal, so the ionic model says: giant lattice of Al3+ and Cl−, melting point in the hundreds or thousands of degrees, conducts when molten. The reality is that AlCl3 turns to vapour at around 180–190 °C and is a poor conductor. A giant ionic lattice does not behave like that.
So what went wrong? The Al3+ ion is tiny and carries three positive charges, which makes it an extremely strong puller of electron density. It drags the outer electron cloud of each chloride ion back towards itself. The electrons never fully leave aluminium’s control — they end up somewhere between transferred and shared. Chemists call that covalent character, and it is why the bonding in AlCl3 is better described as polar covalent than as ionic.
The general rule: small, highly charged cations distort nearby anions, and large anions are easy to distort. Both effects push a compound away from pure ionic and towards covalent character.
The two numbers that fix a substance’s place
To stop guessing, chemists use electronegativity — a number for how strongly an atom in a bond pulls the shared electrons towards itself. Two things are done with those numbers, and each one answers a different question.
The two numbers
Δχ = χA − χB and average χ = ( χA + χB ) ÷ 2
The difference tells you how unevenly the electrons are shared. Big difference means one atom wins the tug of war and the bonding heads towards ionic. Small difference means a fair split.
The average tells you how tightly the pair of atoms holds electrons at all. Two atoms with a low average are both poor at holding electrons, so the electrons wander off into a delocalised sea — that is metallic. Two atoms with a high average both grip hard, so the electrons stay put in a shared pair — that is covalent.
That second idea is the one students skip, and it is the one that makes the triangle make sense. A small difference on its own does not tell you anything. Sodium and sodium have a difference of zero. Chlorine and chlorine also have a difference of zero. One is a soft conductor, the other is a poisonous gas. The average is what separates them.
🧩 How to place any substance
Look up both electronegativities in Section 9 of the data booklet. Never guess them.
Subtract the smaller from the larger to get Δχ. It is always positive.
Add and halve to get the average.
Plot the point as (average, difference) — average along the bottom, difference up the side.
Read off the region you have landed in, and say what that means for the bonding.
The bonding triangle
Plot those two numbers for every substance you can think of and the points fill a triangle. Each corner is one of the three perfect models, and everything real lives inside.
The dashed lines are rough guides, not hard walls. A point just below a border is not a different kind of substance from a point just above it — the whole point of the triangle is that bonding changes gradually.
Reading it is a two-step job. Go along the bottom to find the average, then up to find the difference, then look at where you have landed. Low average and low difference puts you bottom left, in metallic territory. High average and low difference puts you bottom right, covalent. A large difference lifts you towards the apex and the bonding becomes ionic.
Worked examples
WORKED EXAMPLE
Place magnesium oxide on the bonding triangle.
Use χ(Mg) = 1.3 and χ(O) = 3.4 from Section 9 of the data booklet.
Step 1: find the differenceΔχ = 3.4 − 1.3 = 2.1Step 2: find the average(3.4 + 1.3) ÷ 2 = 4.7 ÷ 2 = 2.35Step 3: plot and read
Go to 2.35 along the bottom, then up to 2.1. That is high up near the apex.
Ionic region — MgO is close to a pure ionic compoundand that fits: MgO melts above 2800 °C, exactly what a strong ionic lattice should do
WORKED EXAMPLE
Where does hydrogen chloride sit, and what does that tell you?
Use χ(H) = 2.2 and χ(Cl) = 3.2.
Step 1: the two numbersΔχ = 3.2 − 2.2 = 1.0average = (3.2 + 2.2) ÷ 2 = 2.7Step 2: read the position
High average, middling difference → middle-right of the triangle.
Polar covalent — a shared pair, but pulled towards Clso HCl is a molecule, not a lattice, and the H end carries a small positive charge
WORKED EXAMPLE
Magnesium metal and chlorine gas both have Δχ = 0. Why do they end up at opposite ends of the triangle?
Step 1: both are elements
Every atom is identical, so the difference must be zero. Both sit on the bottom edge.
Step 2: compare the averagesMg: (1.3 + 1.3) ÷ 2 = 1.3Cl: (3.2 + 3.2) ÷ 2 = 3.2Step 3: what a low or high average means
Mg atoms hold their outer electrons weakly, so the electrons delocalise. Cl atoms grip hard, so the electrons stay in a shared pair.
Mg lands bottom left (metallic), Cl2 lands bottom right (covalent)the difference tells you how fair the sharing is; the average tells you whether there is any sharing at all
💡 Exam tip
Always take electronegativity values from Section 9 of the data booklet and quote them in your answer. Examiners give credit for the numbers you used.
Write your point as (average, difference) in that order — it matches the axes and stops you plotting it upside down.
You are never asked to memorise the triangle, only to use it. Practise reading it rather than redrawing it.
If a question says “discuss the bonding in X”, the marks are usually for saying it is intermediate between two models, not for picking one.
Round sensibly. An average of 2.35 does not need three decimal places, and the borders on the triangle are approximate anyway.
For an element, say Δχ = 0 explicitly. It is a free mark that students often leave out.
⚠ Common mix-up
Using the difference on its own. A difference of zero could be a metal or a non-metal. Without the average you have not said anything.
Getting the axes the wrong way round. Difference goes up, average goes across. Plotting (2.1, 2.35) instead of (2.35, 2.1) will land you in the wrong region.
Subtracting in the wrong order. Δχ is always taken as a positive number — larger minus smaller.
Treating the borders as exact. The whole idea is a continuum. A compound sitting on a line is genuinely part-and-part.
Saying “metal + non-metal so it must be ionic”. That rule of thumb is what AlCl3 breaks. Use electronegativity, not the periodic table position.
Confusing electronegativity with electron affinity or ionisation energy. Electronegativity only describes an atom that is already in a bond.
Up next: Bonding and Physical Properties — now that you can place a substance on the triangle, we use that position to predict melting points, conductivity and solubility, and see the one thing the triangle cannot tell you.
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