Why the three bonding boxes are not enough

A bonding model is a simplified description that explains bonding and predicts properties by focusing on the most important particles and forces. Ionic, covalent and metallic bonding work very well as 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 is usually more useful than asking, “Is it ionic or covalent?” as though there are 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. Treat the corners as limiting models. The sides and the area inside 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. When you explain a material, start with the particles present, the forces between them, and whether any charged particles are mobile.

A brittle material is a material that fractures rather than deforms when a force is applied. Ionic crystals behave like this because shifting layers can place like charges 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: high melting point suggests strong bonding; conductivity suggests mobile charged particles; brittleness suggests a lattice that cannot deform easily. Problems start 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.

This is why period 3 oxides make such a good linking example. Across the period, the oxides move from more ionic, basic oxides through amphoteric behaviour to covalent, acidic oxides. Their properties track a trend in bonding, rather than a sudden jump from one perfect category to another.

Locating a binary compound from electronegativity

Where a binary compound sits in the bonding triangle depends on how much ionic, covalent and metallic character the bond has. In the IB course, this judgement comes from electronegativity values in 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:

$\Delta \chi$ mainly points to ionic versus covalent character. If $\Delta \chi$ is large, the electron pair is attracted very unevenly, so the bond has more ionic character. $\bar{\chi}$ helps you tell metallic character apart from covalent character. A low mean electronegativity links to metallic character; a high mean electronegativity links to covalent character.

For an element such as $F_2$ or Si, both atoms have the same electronegativity, so $\Delta \chi = 0$. The value of $\bar{\chi}$ is just the electronegativity of that element. For a compound such as $BaI_2$, use the Ba–I bond; the formula ratio doesn’t move the plotted position. In the same way, $C=O$ in $CO_2$ and $C\equiv O$ in $CO$ are placed using the electronegativities of C and O. Bond order is not used for this diagram.

You do not need to calculate percentage ionic character. If a diagram shows percentage regions, read them qualitatively: “mostly ionic”, “polar covalent with some ionic character”, or “between metallic and covalent” is the sort of interpretation expected.

Predicting properties from position

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

A substance near the ionic region will probably have a high melting point, be brittle, conduct poorly as a solid, and conduct when molten or aqueous if its ions are mobile. Near the covalent molecular region, a substance is more likely to have low electrical conductivity and, depending on molecular size and intermolecular forces, a lower melting point. Between the metallic and covalent regions, a substance may behave like a metalloid: lustrous, brittle and semiconducting.

Silicon is a useful example. It falls 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 give unexpected properties. 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

Look at 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 does not match a simple ionic model.

This is where the triangle is useful. 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 has a combination of 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 better than the pure metal would.

It is more accurate to describe alloys as mixtures, not compounds. Their composition can vary across a range, and the elements are not present in one fixed ratio as they are in a chemical compound. Steel, for example, can contain different proportions of carbon and other elements and still be called steel.

Non-directional bonding and changed properties

A non-directional bond is a bonding interaction that acts in many directions, rather than along one fixed line between two specific atoms. Metallic bonding is non-directional because metal cations are attracted to a sea of delocalized electrons. Pure metals are malleable for this reason: layers of metal ions can slide, while the electron sea keeps holding 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. This disruption can also lower electrical conductivity, because delocalized electrons are scattered more as they move through the less regular structure.

Alloying can improve corrosion resistance and change melting point too. Think of it as small-scale materials design: keep the useful metallic bonding, then adjust the structure by adding selected atoms.

Common alloy examples

You don't 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 its historical use in tools, coins and sculptures. Brass is an alloy based mainly on copper and zinc. It is workable and useful acoustically, so it appears 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 principle applies to carbon steels. Adding carbon to iron makes the lattice harder to deform. If data are supplied, such as hardness against carbon content, interpret 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.

A polymerization reaction joins many monomers into a polymer chain. That chain may contain hundreds or thousands of repeating sections, so drawing the full molecule is usually pointless.

A repeating unit is the smallest group of atoms that repeats along a polymer chain. In structures, it is shown inside square brackets, with bonds crossing the brackets to show the chain continues. The subscript n is used in polymer structures, where n is the number of repeating units in one polymer molecule (dimensionless). In real samples, n is usually large and not identical 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 common classification uses the reaction type that forms 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.

The 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. Poly(chloroethene), by contrast, 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.

This is a lovely materials-science lesson: 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 losing any small molecule. Addition polymerization is a polymerization reaction where the $\pi$ 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 be given, or you may need to deduce the monomer from a section of polymer.

For ethene forming polyethene:

Look carefully at the change: 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, $CH_2=CHCH_3$, the repeating unit is $[-CH_2-CH(CH_3)-]_n$. For chloroethene, $CH_2=CHCl$, it is $[-CH_2-CHCl-]_n$. For tetrafluoroethene, $CF_2=CF_2$, it is $[-CF_2-CF_2-]_n$.

To go backwards from polymer to monomer, reverse the steps: 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, by contrast, is a long-chain molecular solid; its overall intermolecular attractions are much stronger because there are many contact points along each chain. Both contain covalent bonds, but changing the size 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 each other 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 ends up in the polymer, and no small molecule is eliminated. Therefore, addition polymerization has $100\%$ atom economy for the polymer product.

Condensation polymerization

A condensation polymer forms when monomers react through their functional groups, releasing a small molecule each time a new linkage forms. Condensation polymerization is a polymerization reaction in which bifunctional or multifunctional monomers join with elimination of a small molecule such as water or hydrogen chloride.

A functional group is a specific atom or group of atoms in an organic molecule that gives the molecule characteristic reactions. In condensation polymerization, each monomer needs reactive functional groups at two positions, usually one at each end, so the chain can continue to grow.

Useful pairs include:

• carboxyl group, –COOH, with hydroxyl group, –OH, forming an ester linkage and water;
• carboxyl group, –COOH, with amino group, $-NH_2$, forming an amide linkage and water;
• acyl chloride group, –COCl, with hydroxyl group, –OH, forming an ester linkage and hydrogen chloride;
• acyl chloride group, –COCl, with amino group, $-NH_2$, forming an amide linkage and hydrogen chloride.

An ester linkage is a covalent linkage containing –COO–, made when a carboxyl derivative reacts with a hydroxyl group. An amide linkage is a covalent linkage containing –CONH–, made when a carboxyl derivative reacts with an amino group. In proteins, an amide linkage is also called a peptide bond.

Polyesters from diacids and diols

A polyester is a condensation polymer with many ester linkages in its backbone. It commonly forms from a dicarboxylic acid and a diol.

A dicarboxylic acid has two –COOH groups. A diol has two –OH groups. One –COOH reacts with one –OH to form –COO– and $H_2O$. Because both monomers still have another reactive end, the reaction can repeat and build a chain.

When drawing the repeating unit of a polyester from given monomers, remove H from each alcohol end and OH from each carboxyl end to make water. Then connect the remaining O to the carbonyl carbon. The repeating unit must show the ester linkages and the carbon skeletons from both monomers.

Polyamides from diacids and diamines

A polyamide is a condensation polymer with many amide linkages in its backbone. Nylon-6,6 is the standard example: it forms from a dicarboxylic acid and a diamine. A diamine has two $-NH_2$ groups.

To draw a polyamide repeating unit from given monomers, connect the carbonyl carbon of the acid-derived part to nitrogen from the amine-derived part, forming –CONH–. Each amide linkage formed from –COOH and $-NH_2$ releases water. If an acyl chloride is used instead of a carboxylic acid, hydrogen chloride is released.

Same-monomer condensation

Some monomers carry two different functional groups in the same molecule. A hydroxycarboxylic acid contains both –OH and –COOH, so it can polymerize with itself to form a polyester. An amino acid contains both $-NH_2$ and –COOH, so it can polymerize with itself to form a polyamide-like chain called a polypeptide.

A biological macromolecule is a large molecule produced by living organisms, such as a protein, nucleic acid or polysaccharide. In this course, the key generalization is that biological macromolecules form by condensation reactions and break down by hydrolysis.

Hydrolysis: the reverse reaction

Hydrolysis is a reaction in which water breaks a covalent linkage, often reversing a condensation reaction. In biological systems, proteins hydrolyse at peptide bonds, polysaccharides hydrolyse at glycosidic linkages, and nucleic acids hydrolyse at linkages in their backbones.

A glycosidic linkage is a covalent linkage between sugar units formed by condensation of hydroxyl groups. Polysaccharides such as starch and cellulose form when monosaccharide units join by condensation, releasing water; hydrolysis adds water back across the linkage and breaks the polymer into smaller carbohydrate units.

So water is not just a solvent in biology. It acts as a reactant in the breakdown of many biological polymers, while removal of water is associated with building them.