What you'll learn
- How elements (one type of atom) and compounds (different elements chemically joined) are classified by structure and bonding.
- Why ionic, simple molecular, giant covalent and metallic substances have different physical properties.
- How diamond, graphite, graphene and fullerenes are linked to their carbon structures.
- Why chemical models are useful, but always limited.
The big idea: structure causes properties
A substance is a material with a particular chemical identity, such as sodium chloride, oxygen or copper.
Structure means how the particles are arranged. Bonding means the attractions that hold particles together. A physical property is something you can observe or measure without making a new substance, such as melting point, boiling point, solubility or electrical conductivity.
Solubility means how well a substance dissolves in a solvent. In GCSE chemistry, the solvent is often water. Aqueous means dissolved in water, shown by the state symbol (aq).
Particles and bonding words
- An ion is an atom or group of atoms with an electric charge.
- A molecule is a small group of atoms joined by covalent bonds.
- A covalent bond is a strong bond formed when atoms share a pair of electrons.
- A lattice is a regular, repeating arrangement of particles.
- Delocalised electrons are electrons that can move through a structure instead of belonging to just one atom or one bond.
The property chain
To explain a property, link structure → bonding/forces → particle movement or energy needed → observed property.
The diagram below compares the four main structure types. For giant covalent substances, “no mobile charged particles” is the usual pattern, but graphite and graphene are important carbon exceptions.
The four main types of substance
1. Ionic substances
An ionic substance contains positive and negative ions in a giant ionic lattice. Ionic compounds usually form between metals and non-metals, such as sodium chloride, NaCl.
Ionic compounds have:
- High melting points and boiling points because strong electrostatic attractions act between oppositely charged ions throughout the lattice.
- No electrical conductivity as solids because the ions are charged but fixed in position.
- Electrical conductivity when molten or in aqueous solution because the ions can move and carry charge.
- Often good solubility in water, although not every ionic compound dissolves well.
Molten means melted into a liquid.
Identifying an ionic substance from its properties
A substance has a high melting point. It does not conduct electricity as a solid, but it conducts when molten and when dissolved in water.
- A high melting point suggests strong attractions between particles, so the substance is unlikely to be simple molecular.
- The solid does not conduct, so its charged particles are not free to move in the solid state.
- It conducts when molten and aqueous, so mobile ions must be present in those states. This matches a giant ionic lattice.
2. Simple molecular covalent substances
A simple molecular substance is made of separate small molecules. Examples include oxygen, O₂, carbon dioxide, CO₂, water, H₂O, and methane, CH₄.
Simple molecular substances usually have:
- Low melting points and boiling points because only weak intermolecular forces need to be overcome.
- Poor electrical conductivity because they do not contain mobile ions or delocalised electrons.
- Often low solubility in water, though there are exceptions such as sugar and ethanol. If a molecular substance dissolves without forming ions, the solution still does not conduct well.
Intermolecular forces
Intermolecular forces are weak attractions between separate molecules. They are not the same as the strong covalent bonds inside molecules.
Weak forces, not weak covalent bonds
When a simple molecular substance melts or boils, the covalent bonds inside the molecules usually stay intact. The weak intermolecular forces between molecules are overcome.
Explaining a low boiling point
Carbon dioxide is a gas at room temperature.
- Carbon dioxide is made of small, separate CO₂ molecules.
- Boiling separates the molecules from each other, so only weak intermolecular forces need to be overcome.
- Little energy is needed compared with ionic or giant covalent structures, so carbon dioxide has a low boiling point.
3. Giant covalent substances
A giant covalent substance has atoms joined by strong covalent bonds in a huge repeating network. Examples include diamond, graphite, graphene and silicon dioxide.
Giant covalent substances usually have:
- Very high melting points and boiling points because many strong covalent bonds must be broken.
- No solubility in water because the network is too strongly bonded to separate into particles.
- Poor electrical conductivity, unless they contain delocalised electrons, as graphite and graphene do.
Carbon structures: same element, different arrangements
Diamond and graphite are different forms of carbon and are examples of giant covalent substances.
Allotrope
An allotrope is a different structural form of the same element. Diamond, graphite, graphene and fullerenes are allotropes of carbon.
The diagram below shows how changing the bonding arrangement changes the properties of carbon.
Diamond
In diamond, each carbon atom forms four covalent bonds to other carbon atoms. This makes a rigid three-dimensional giant covalent network.
Diamond is:
- Very hard, because strong covalent bonds hold the structure in all directions.
- Used in cutting tools, because it can scratch and cut many other materials.
- A poor electrical conductor, because it has no delocalised electrons.
Graphite
In graphite, each carbon atom forms three covalent bonds in flat hexagonal layers. One electron from each carbon atom is delocalised.
Graphite is:
- A good electrical conductor, because delocalised electrons can move through the layers.
- Used as an electrode. An electrode is a conductor that carries current into or out of a chemical system.
- Used as a lubricant. A lubricant reduces friction, and graphite works because its layers can slide over each other.
Choosing graphite for an electrode
A student must choose between diamond and graphite for an electrode.
- An electrode must conduct electricity, so it needs mobile charged particles.
- In graphite, each carbon atom bonds to three others, leaving delocalised electrons that can move.
- In diamond, each carbon atom uses all four outer electrons in covalent bonds, so there are no mobile electrons. Graphite is the better choice.
Graphene and fullerenes
Graphene is a single layer of graphite, just one carbon atom thick. Each carbon atom bonds to three others in a hexagonal sheet. Graphene is very strong and conducts electricity because it has delocalised electrons.
Fullerenes are carbon molecules with hollow shapes. C60 is a roughly spherical molecule made from 60 carbon atoms. Each carbon atom bonds to three others. Because C60 molecules are separate molecules, the forces between molecules are much weaker than the covalent bonds inside each molecule.
Simple polymers: poly(ethene)
Polymer
A polymer is a very large molecule made when many small molecules, called monomers, join together in a chain.
A simple polymer such as poly(ethene) contains long chains of carbon atoms. The carbon atoms are joined by covalent bonds, with hydrogen atoms attached to the chain.
A simplified equation for forming poly(ethene) is:
n CH2=CH2(g)→(CH2−CH2)n(s)n\,\text{CH}_2{=}\text{CH}_2\text{(g)} \to \left(\text{CH}_2{-}\text{CH}_2\right)_n\text{(s)}nCH2=CH2(g)→(CH2−CH2)n(s)Here, nnn means a very large number of ethene molecules join together. Poly(ethene) is not a giant covalent lattice like diamond; it is made of very large separate molecules.
Metallic substances
A metallic substance contains positive metal ions arranged in layers, surrounded by delocalised electrons. The attraction between the positive metal ions and the delocalised electrons is called metallic bonding.
Metals are usually:
- Good electrical conductors, because delocalised electrons can move through the solid.
- Malleable, meaning they can be hammered or rolled into shape.
- Shiny solids with high melting points.
- High density, meaning a large mass in a small volume.
Explaining malleability and conductivity
Copper is malleable and conducts electricity.
- Copper has positive metal ions in layers with delocalised electrons between them.
- When copper is hammered, the layers of ions can slide over each other while the metallic bonding still holds the structure together.
- The delocalised electrons can move through the metal and carry charge, so copper conducts electricity.
Most means not all
Most metals are shiny solid conductors, and most non-metals have low boiling points and are poor conductors. But there are exceptions: graphite is a non-metal that conducts, and mercury is a metal that is liquid at room temperature.
Limits of models and diagrams
Chemical models help you picture particles that are far too small to see directly, but every model leaves something out.
- Dot-and-cross diagrams show outer-shell electrons and where they came from, but they do not show true electron positions, real sizes or the full 3D structure.
- Ball-and-stick models show which atoms are bonded and the shape of a molecule, but atoms are not really balls joined by rods. The gaps are exaggerated.
- 2D diagrams are useful on paper, but they can hide 3D bonding. Diamond is a 3D network, while graphite has layers.
- 3D models show shape better, but they can make it harder to count atoms or see repeated lattices clearly.
Models are not reality
Do not describe a model as if it is the actual substance. For example, a ball-and-stick model shows bonds as sticks, but real chemical bonds are attractions between particles.
In the exam
- Start property explanations with the structure: ions, molecules, giant covalent network or metal lattice.
- For conductivity, always ask: “Are there mobile charged particles?”
- For melting and boiling points, say which forces or bonds are overcome, and whether they are strong or weak.
Check yourself
- Why does solid sodium chloride not conduct electricity, but molten sodium chloride does?
- Why is graphite suitable for electrodes, while diamond is not?
- What is one limitation of a dot-and-cross diagram?
