IB Chemistry SLTopic 2 — From Bonding Models to MaterialsPaper 1 & 2Core skill~11 min read
Bonding and Physical Properties
Now put the triangle to work. Plot a substance, read off how much ionic or covalent character it has, and you can explain why two compounds that look similar on paper behave completely differently in the lab.
📚 What you need to know
Plot every substance as (average electronegativity, electronegativity difference).
Elements sit on the x-axis because their Δχ is zero.
Read horizontally across from a point to the right-hand scale to estimate % ionic character.
More ionic character → higher melting point, more likely to dissolve in water, conducts when molten.
More covalent character → lower melting point, more volatile, does not conduct.
You will never be asked to calculate an exact percentage — only to compare substances.
Plotting a compound and its elements
Sodium chloride is the classic starting point, because you can plot all three species and see them separate:
Sodium (Na) — electronegativity 0.9. As an element, Δχ = 0, so it plots at (0.9, 0) — bottom left, in the metallic corner.
Chlorine (Cl2) — electronegativity 3.2. Also an element, so (3.2, 0) — bottom right, purely covalent.
Sodium chloride (NaCl) — average 2.05, difference 2.3, so (2.05, 2.3) — high up near the apex, about 75% ionic.
Sodium sits in the metallic corner, chlorine in the covalent corner, and the compound they form lands near the ionic apex.
Look at what happened. Two elements that sit at opposite bottom corners combine to give something near the top. That is the triangle showing you, visually, why a metal and a non-metal usually give an ionic compound — and NaCl’s high melting point and conductivity when molten follow directly.
WORKED EXAMPLE
Use electronegativity values to find the position on the bonding triangle of (a) phosphorus, (b) caesium iodide, (c) brass, a copper–zinc alloy.
(a) Phosphorus — an element, χ = 2.2Same atom both sides, so the difference is zero.(2.2, 0) — bottom middle, 100% covalent(b) Caesium iodide — Cs = 1.0, I = 2.7χ = (1.0 + 2.7) ÷ 2 = 1.85Δχ = 2.7 − 1.0 = 1.7(1.85, 1.7) — in the ionic region(c) Brass — Cu = 1.9, Zn = 1.6χ = (1.9 + 1.6) ÷ 2 = 1.75Δχ = 1.9 − 1.6 = 0.3(1.75, 0.3) — low down and to the left, on the metallic borderWhich is exactly right — brass is an alloy and behaves like a metal.
Estimating percentage ionic character
The scale running down the right-hand side of the triangle is the useful part. Find your point, run your eye horizontally across, and read off roughly how ionic the bonding is. Height on the triangle is what matters here — the further up, the more ionic.
Aluminium chloride and aluminium oxide make the comparison beautifully. Aluminium is 1.6, chlorine 3.2 and oxygen 3.4:
Two points only slightly apart on the triangle — but the melting points differ by nearly 1900 °C.
Turning position into properties
That 10% difference in ionic character does not sound like much. The properties tell a different story:
AlCl3
Al2O3
Position
(2.4, 1.6)
(2.5, 1.8)
Ionic character
~50%
~60%
Melting point
192 °C
2072 °C
What holds it together
Largely covalent molecules with weak forces between them
A giant ionic lattice of strong attractions
More ionic character means more of the structure is held by strong electrostatic attractions in a giant lattice, so far more energy is needed to melt it. The same reasoning predicts solubility in water and conductivity when molten — both go up as ionic character goes up.
🧩 Using the triangle in an exam
Look up both electronegativities in Section 9 of the data booklet.
Work out the average (x) and the difference (y).
Plot the point on the triangle in Section 17.
Name the region it lands in, and read across for rough % ionic character if you need to compare.
Link that back to properties: more ionic → higher melting point, soluble in water, conducts when molten.
WORKED EXAMPLE
Magnesium chloride melts at 714 °C but aluminium chloride melts at 192 °C. Use electronegativity values (Mg = 1.3, Al = 1.6, Cl = 3.2) to explain the difference.
MgCl₂: Δχ = 3.2 − 1.3 = 1.9AlCl₃: Δχ = 3.2 − 1.6 = 1.6MgCl₂ sits higher on the triangle, so it has more ionic character.More ionic character → more of a giant lattice of strong electrostatic attractionsMgCl₂ needs far more energy to meltAlCl₃ is largely covalent, so only weak forces between molecules have to be overcome.
💡 Exam tip
You are never asked for an exact percentage. Compare qualitatively: “X has more ionic character than Y”.
Quote the numbers you calculated in your answer. “Δχ is larger for MgCl2” is a mark; “it’s more ionic” alone often isn’t.
Finish the chain: more ionic → stronger attractions → higher melting point. Don’t stop halfway.
Both data booklet sections are allowed in the exam — know where Section 9 and Section 17 are before you sit it.
⚠️ Common mix-up
% ionic character is read from the height (Δχ), not from the horizontal position. The x-axis tells you metallic versus covalent, not how ionic something is.
50% ionic does not mean “half the bonds are ionic”. It means every bond has roughly equal ionic and covalent character.
Don’t assume a high melting point automatically means ionic — giant covalent structures like SiO2 melt even higher.
The triangle uses electronegativity, not position in the periodic table. Two compounds of the same group can land in different regions.
Up next: Alloys and Their Properties — what happens when you mix metals together, and why the mixture is usually more useful than either pure metal.
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