Bonding isn't three neat boxes but a continuum — and that idea explains alloys, plastics and the materials all around you.
Theme · StructureStructure 2.4
This is the Standard Level lesson, covering S2.4.1–S2.4.5. Taking Chemistry at Higher Level? The HL version adds condensation polymers (polyesters and polyamides).
👆 Plot compounds on the bonding triangle · build addition polymers
1. Bonding is a continuum S2.4.1–2
Ionic, covalent and metallic bonding are not separate boxes — they blend into one another. The bonding triangle maps any compound using two numbers from electronegativity (EN): the difference in EN (up the triangle → more ionic) and the average EN (right → more covalent, left → more metallic). Pick a substance and see where it lands:
Bonding triangle plotter
Pick a substanceIts position is set by the average electronegativity (x) and the electronegativity difference (y).
2. Alloys S2.4.3
An alloy is a mixture of a metal with one or more other metals or non-metals (e.g. steel = iron + carbon; bronze = copper + tin). The added atoms are a different size, so they disrupt the neat layers and stop them sliding — making the alloy harder and stronger than the pure metal, while the delocalised electrons still hold it together.
Pure metal — layers slide (soft)
Alloy — bigger atoms block sliding (hard)
3. Polymers & addition polymerisation S2.4.4–5
Polymers are giant molecules (macromolecules) built from many small repeating units called monomers. In addition polymerisation, monomers each contain a C=C double bond; the double bond opens up and the monomers join into a long chain with no atoms left over. Pick a monomer to see its repeating unit:
Going to HL? The HL lesson adds condensation polymers — polyesters and polyamides, where two functional groups react and a small molecule (usually water) is released each time a link forms. → Open the HL version
Common mistakes examiners see
What two quantities position a compound on the bonding triangle?✗ Melting point and density.✓ The difference in electronegativity (vertical) and the average electronegativity (horizontal).
Why is an alloy usually harder than the pure metal?✗ The extra atoms form stronger covalent bonds.✓ The different-sized atoms distort the layers so they can no longer slide over each other easily.
What happens to the C=C bond during addition polymerisation?✗ It stays as a double bond in the polymer.✓ The double bond opens (becomes a single bond) so each carbon can bond to the next monomer — no atoms are lost.
Is a polymer a giant covalent network like diamond?✗ Yes, both are giant structures.✓ No — a polymer is made of long molecules held to each other by intermolecular forces; diamond is one continuous covalent network.