Why the three bonding boxes are not enough

A bonding model is a simplified description used to explain bonding and predict properties by focusing on the main particles and forces involved. Ionic, covalent and metallic bonding are very useful models, but real substances don’t always fit neatly into one box.

A bonding continuum is a range of bonding behaviour in which substances show different degrees of ionic, covalent and metallic character. It’s usually more helpful than asking, “Is it ionic or covalent?” as though those are the only two possible answers. Aluminium chloride, for example, has important ionic and covalent features; metalloids such as silicon show both covalent and metallic behaviour.

A bonding triangle is a triangular diagram that represents bonding character using the three idealized corners: ionic, covalent and metallic. These corners are limiting models. The sides and the inside of the triangle show mixtures of bonding character. The data booklet provides this triangular diagram, so you are expected to use it rather than memorize its exact layout.

Using bonding models to explain properties

Properties come from structure. To explain a material, start with the particles present, the forces between them, and whether any charged particles can move.

A brittle material is a material that fractures rather than deforms when a force is applied. Ionic crystals are brittle because, if layers shift, like charges can end up next to each other, causing repulsion and fracture. A malleable material is a material that can be hammered or pressed into shape because layers of particles can move while bonding is maintained; metals are the classic example.

A corrosion reaction is a chemical reaction between a material and its environment that damages the material. Rusting of iron is a familiar example, but corrosion covers more than rusting.

Why classification has limits

Discrete bonding categories are useful because they help us spot patterns quickly: a high melting point suggests strong bonding; conductivity points to mobile charged particles; brittleness suggests a lattice that cannot deform easily. The problem comes when the labels are treated as absolute. Some covalent network solids conduct, some molecular covalent polymers are tough, some ionic compounds have substantial covalent character, and many useful materials deliberately combine bonding types.

Period 3 oxides make a good linking example for this reason. Across the period, the oxides move from more ionic, basic oxides through amphoteric behaviour to covalent, acidic oxides. Their properties show a trend in bonding rather than a sudden jump from one perfect category to another.

Locating a binary compound from electronegativity

A binary compound’s position in the bonding triangle comes from how much ionic, covalent and metallic character the bond has. In the IB course, the judgement is made using electronegativity values from the data booklet.

Electronegativity is a dimensionless measure of how strongly an atom in a bond attracts the shared electron pair. For a bond between atoms A and B, calculate:

Δχ = |χ_A − χ_B|, where Δχ is the electronegativity difference (dimensionless), χ_A is the electronegativity of atom A (dimensionless) and χ_B is the electronegativity of atom B (dimensionless).

χ̄ = (χ_A + χ_B) / 2, where χ̄ is the mean electronegativity of the two bonded atoms (dimensionless).

Δχ mainly shows the balance between ionic and covalent character. When Δχ is large, the electron pair is attracted much more strongly by one atom, so the bond has more ionic character. χ̄ helps you tell metallic character apart from covalent character. Low mean electronegativity is linked with metallic character; high mean electronegativity is linked with covalent character.

For an element such as F₂ or Si, both atoms have the same electronegativity, so Δχ = 0. The value of χ̄ is just the electronegativity of that element. For a compound such as BaI₂, use the Ba–I bond; the formula ratio does not move the plotted position. In the same way, C=O in CO₂ and C≡O in CO are plotted from the C and O electronegativities. Bond order is not used for this diagram.

You don’t need to calculate percentage ionic character. If a diagram labels percentage regions, treat them qualitatively: “mostly ionic”, “polar covalent with some ionic character”, or “between metallic and covalent” is the kind of interpretation expected.

Predicting properties from position

Once you have plotted the position, use the nearby bonding models to predict properties.

A substance near the ionic region is likely to have a high melting point, be brittle, conduct poorly as a solid, and conduct when molten or aqueous if ions are mobile. A substance near the covalent molecular region is more likely to show low electrical conductivity and, depending on molecular size and intermolecular forces, a lower melting point. A substance between the metallic and covalent regions may behave like a metalloid, with lustre, brittleness and semiconducting properties.

Silicon is a useful example. It sits between metallic and covalent character. Its atoms form a covalent network, so it is brittle, but it also shows some metallic-style behaviour: it is lustrous and acts as a semiconductor rather than a typical insulator.

Mixed bonding can catch you out. A compound with significant ionic and covalent character may not behave like a textbook ionic solid. Aluminium chloride, for example, has enough covalent character to show an unusually low melting point for a substance often introduced as ionic, and it can form molecular species. The bonding triangle is a model, not a magic property machine.

Comparing materials with varying bonding character

Consider two families of binary compounds. Potassium halides are strongly ionic: as the halide ion gets larger from fluoride to iodide, electrostatic attraction weakens and melting points tend to decrease. Silver halides have greater covalent character. Their melting behaviour is affected more by polarizability and intermolecular attractions, so the trend is not the same as in a simple ionic model.

This is the sort of comparison the triangle handles well. It helps you connect bonding character to properties, but it does not replace careful structural reasoning.

Why composites have unusual properties

A composite material is a material made from two or more distinct components that remain separate on the microscopic scale but act together to give improved properties. Reinforced concrete is a good example: concrete contains ionic and covalent components that resist compression well, while steel bars provide tensile strength and some flexibility through metallic bonding. The final material combines properties that none of the components has alone.

What an alloy is

An alloy is a mixture where a metal is combined with one or more other elements, either metals or non-metals, while metallic bonding through delocalized electrons is retained. Alloys usually still show metallic properties, such as electrical conductivity and lustre, but their mechanical and chemical properties can suit a particular use much better than the pure metal would.

It is more accurate to call alloys mixtures rather than compounds. Their composition can vary across a range, and the elements in them are not locked into one fixed ratio as they would be in a chemical compound. Steel, for example, can contain different proportions of carbon and other elements while still being called steel.

Non-directional bonding and changed properties

A non-directional bond is a bonding interaction that acts in many directions, not just along one fixed line between two specific atoms. Metallic bonding is non-directional because metal cations are attracted to a sea of delocalized electrons. This helps explain why pure metals are malleable: layers of metal ions can slide past each other, while the electron sea still holds the structure together.

In an alloy, atoms or ions with different sizes disturb the regular arrangement of the metallic lattice. Apply a force, and the layers no longer slide as easily. Many alloys are therefore harder and stronger than the pure metal. The disruption can also reduce electrical conductivity, since delocalized electrons are scattered more as they move through the less regular structure.

Alloying can also improve corrosion resistance and alter melting point. It’s materials design on a small scale: keep the useful metallic bonding, then tune the structure by adding selected atoms.

Common alloy examples

You do not need to memorize detailed compositions, but you should be able to discuss familiar examples.

Bronze is an alloy based mainly on copper and tin. It is harder than copper and resists corrosion well, which explains why it was historically used in tools, coins and sculptures. Brass is an alloy based mainly on copper and zinc. It is workable and acoustically useful, so it is used in musical instruments and fittings. Stainless steel is an iron-based alloy containing chromium; chromium forms a thin protective oxide layer that helps prevent rusting, making it useful for kitchen and medical equipment.

The same idea applies to carbon steels. Adding carbon to iron makes the lattice harder to deform. If data are supplied, such as hardness against carbon content, read the trend by linking the independent variable to lattice disruption and mechanical strength. The Vickers hardness test gives a hardness value, often reported as HV, and larger HV values mean a harder material.

Polymer chains and repeating units

A polymer is a macromolecule made from many repeating subunits joined by covalent bonds. A macromolecule is a molecule with a very large relative molecular mass because it contains many atoms covalently joined together. A monomer is a small molecule that can react with other monomer molecules to form a polymer.

In a polymerization reaction, many monomers join together to make a polymer chain. That chain may have hundreds or thousands of repeating sections, so there’s rarely any point in drawing the entire molecule.

A repeating unit is the smallest group of atoms that repeats along a polymer chain. It is shown inside square brackets, with bonds crossing the brackets to show that the chain continues. Polymer structures use the subscript n, where n is the number of repeating units in one polymer molecule (dimensionless). In real samples, n is usually large, and it isn’t exactly the same for every chain.

Polymers can be grouped by source: natural, synthetic or semi-synthetic. Natural polymers include cellulose, starch, proteins, DNA, silk and natural rubber. Synthetic polymers include polyethene, poly(chloroethene), polystyrene, nylon, PET and PTFE. Another way to classify them is by the reaction type that makes them: addition or condensation.

Common properties of plastics from structure

A plastic is a synthetic polymer material that can be shaped when soft and then used as a solid. Most plastics are covalent molecular materials. Strong covalent bonds hold the atoms within each chain, while intermolecular forces, and sometimes crosslinks, hold separate chains together.

This structure explains several common properties:

• Plastics are usually electrical insulators because they do not contain mobile ions or delocalized electrons.
• They often have low thermal conductivity because energy is not transferred efficiently through mobile electrons as it is in metals.
• They can be strong and tough because the chains are long and may become tangled.
• They often have higher melting or softening points than small covalent molecules because many intermolecular attractions act along each long chain.
• They are often low-density materials, which is one reason they are useful in packaging and transport.

Exact properties depend on chain length, side groups, chain packing, intermolecular forces and crosslinking. Polyethene is non-polar and held mainly by London dispersion forces, while poly(chloroethene) has polar C–Cl bonds and stronger dipole-dipole attractions between chains. In polystyrene, bulky phenyl side groups make chain movement difficult, so the material is more brittle.

Natural polymers: same monomer, different structure, different properties

Cellulose and starch are both natural glucose polymers, but they behave very differently. Cellulose forms straight chains that pack closely and hydrogen bond strongly, giving strong fibres found in plant cell walls and cotton. Starch has a more branched structure, so its chains do not pack as tightly and it is easier to break down during digestion.

There’s a neat materials-science point here: the monomer matters, but the way monomers are linked matters just as much.

Biodegradable plastics and environmental structure

Biodegradation is the breakdown of a material by microorganisms into smaller molecules. Many plastics resist biodegradation because their carbon-chain backbones are unreactive, they lack functional groups that enzymes can attack, and crosslinked networks leave few entry points for microorganisms.

A biodegradable plastic is a plastic designed to be broken down by microorganisms under suitable conditions. Structural features that can help include shorter chains, higher surface area, hydrolysable linkages such as ester groups, carbohydrate or starch components, light-sensitive groups, and functional groups that microorganisms can bind to. Disposal conditions still matter: low oxygen, low light or cold environments can slow degradation.

Microplastics are not the same as biodegradation. A microplastic is a plastic fragment smaller than 5 mm formed by physical breakdown of larger plastic items. The polymer may remain chemically unchanged, which is why microplastics can persist and enter food chains.

How addition polymerization works

An addition polymer forms when unsaturated monomers join together without eliminating any small molecule. Addition polymerization is a polymerization reaction where the π bond in each carbon-carbon double bond breaks, then new carbon-carbon single bonds link the monomers.

For an alkene addition monomer, the key functional group is the C=C double bond. Alkenes such as ethene, propene, chloroethene and tetrafluoroethene can form addition polymers. You don’t have to learn the monomer structures; in an exam they will either be given, or you may need to deduce the monomer from a polymer section.

For ethene forming polyethene:

n CH₂=CH₂ → [–CH₂–CH₂–]ₙ

Here n is the number of repeating units in a polymer molecule, as introduced earlier. The main change is simple: the C=C double bond in the monomer becomes C–C single bonds in the polymer backbone. Nothing is lost from the monomer.

Drawing repeating units from alkene monomers

Use this reliable method.

1. Find the C=C bond in the monomer.
2. Change C=C into a C–C single bond.
3. Keep every atom or group attached to each carbon on the same carbon.
4. Put square brackets around the two-carbon repeating unit.
5. Draw continuation bonds through the brackets.
6. Add n outside the bracket if representing the polymer.

For propene, CH₂=CHCH₃, the repeating unit is [–CH₂–CH(CH₃)–]ₙ. For chloroethene, CH₂=CHCl, the repeating unit is [–CH₂–CHCl–]ₙ. For tetrafluoroethene, CF₂=CF₂, the repeating unit is [–CF₂–CF₂–]ₙ.

To go backwards from polymer to monomer, reverse the process: take the two-carbon repeating unit, remove the bracket continuation bonds, and put a C=C double bond between the two backbone carbons.

Properties: monomer versus polymer

Ethene is a small molecular gas with low melting and boiling points. Polyethene is a long-chain molecular solid, so the overall intermolecular attractions are much stronger because there are many contact points along each chain. Both contain covalent bonds, but changing the scale of the molecule changes the physical properties dramatically.

Most addition polymers are electrical insulators because they have no mobile charged particles. Their strength and softening temperature depend on side groups and intermolecular forces. Poly(chloroethene), often called PVC, has polar C–Cl bonds, so its chains attract more strongly than non-polar polyethene chains.

Atom economy

Atom economy is the percentage of reactant atoms that become part of the desired product in a chemical reaction. In addition polymerization, every atom in the alkene monomers appears in the polymer and no small molecule is eliminated. Addition polymerization therefore has 100% atom economy for the polymer product.