Bonding, structure and properties of matter

1. Why does a material behave the way it does?

A pencil writes, a copper wire carries current and table salt forms crystals. Why do these materials behave differently? The answer starts with their particles and the forces holding them together.

A chemical bond is a strong attraction that holds particles together. Opposite electric charges attract: positive attracts negative. The scientific name for this is electrostatic attraction.

2. Ionic bonding: electrons are transferred

Imagine tracking the outer electrons as a metal reacts with a non-metal. The metal atoms lose electrons; the non-metal atoms gain electrons. The atoms become charged particles called ions.

• Lose negative electrons → positive ion. There are now more protons than electrons.
• Gain negative electrons → negative ion. There are now more electrons than protons.
• The nucleus stays the same. Protons are not transferred.

The ions formed have a full outer shell, like a noble gas. Sodium changes from 2,8,1 to 2,8. Chlorine changes from 2,8,7 to 2,8,8.

When drawing an ion, put square brackets around it and write its charge at the top right, outside the brackets. Show the gained electrons in the non-metal ion’s full outer shell.

An ionic formula gives the simplest whole-number ratio of ions. This is called the empirical formula. It does not describe a separate molecule.

3. Salt crystals: a giant pattern of ions

Table salt is not made of separate sodium chloride molecules. It contains a giant lattice: a repeating three-dimensional pattern of positive and negative ions.

Each ion attracts nearby ions with the opposite charge. These strong attractions act in all directions through the crystal, not just between one pair of ions.

• Why a high melting point? Many strong ionic attractions must be overcome. This needs a large amount of energy. The boiling point is also high.
• Why no current through a solid? The ions have charge, but they are held in fixed positions. They cannot move through the solid to carry charge.
• Why can a melted salt conduct? Its ions are free to move and carry charge. A salt solution can conduct for the same reason.

4. Covalent bonding: share a pair

In covalent bonding, atoms share electrons. One shared pair makes one covalent bond. The negative electrons are attracted to the positive nucleus of each bonded atom. This strong attraction holds the atoms together.

The overlapping circles below show the atoms’ outer electron shells. A shared pair sits in the overlap, so both atoms count it as part of their outer shell.

Hydrogen has a full first shell with two electrons. The other atoms shown here achieve eight outer-shell electrons by sharing. Inner shells are left out of these covalent diagrams to keep the sharing clear.

A displayed formula uses lines instead of drawing electrons. One line means one shared pair. Double and triple lines mean two and three shared pairs.

5. Boiling: separating molecules, not splitting them

Water, oxygen, nitrogen and methane contain small molecules. There are two different kinds of attraction to keep track of:

• Inside each molecule: strong covalent bonds hold its atoms together.
• Between molecules: much weaker attractions hold neighbouring molecules near each other. These are intermolecular forces.

When a molecular substance melts or boils, the molecules move past one another or move apart. The forces between them are overcome. The strong covalent bonds inside them remain. This is why many small molecular substances have low melting and boiling points.

For comparable molecules, larger molecules have stronger intermolecular forces. More energy is needed to separate them, so their boiling points are generally higher.

Small molecular substances usually do not conduct electricity. They have neither mobile ions nor delocalised electrons to carry charge.

6. Polymers: long chains of repeating units

A plastic bag contains polymers: substances made of very long molecules. Each molecule has a pattern called a repeating unit that repeats many times along the chain.

Strong covalent bonds join the atoms along each chain. Relatively strong intermolecular forces act between the long chains. These forces help make polymers solids at room temperature.

Draw a repeating unit inside square brackets, with bonds continuing through both sides. Put a small n outside: it means the unit repeats a large number of times. For poly(ethene), the unit is [–CH₂–CH₂–]ₙ. Each carbon has four bonds in total.

7. Giant covalent structures: one huge network

In a giant covalent structure, a very large number of atoms are joined in a continuous network by strong covalent bonds.

Diamond, graphite and silicon dioxide (SiO₂) are examples. They are solids with very high melting points. Breaking up their networks means overcoming many strong covalent bonds, which needs a large amount of energy.

In silicon dioxide, each silicon atom bonds to four oxygen atoms. Each oxygen links two silicon atoms. The overall ratio is one silicon to two oxygen atoms, giving SiO₂. The solid does not contain separate SiO₂ molecules.

Most giant covalent substances do not conduct electricity because they have no mobile charged particles. Graphite and graphene are exceptions: they have electrons that can move through the structure. You will meet them in the carbon section.

8. Metals: electrons on the move

Why can a copper wire carry current? A metal has a giant regular arrangement of positive metal ions surrounded by delocalised electrons.

Delocalised means the electrons are not tied to one atom or one bond. They can move through the structure. The attraction between these negative electrons and the positive metal ions is metallic bonding.

• Usually high melting and boiling points: strong metallic attractions need a large amount of energy to overcome.
• Conducts electricity: delocalised electrons move through the solid or liquid and carry charge.
• Conducts thermal energy: mobile electrons transfer energy through the structure.
• Can bend and be shaped: layers in a pure metal slide past each other while metallic bonding still holds the material together.

An alloy is a mixture containing a metal and one or more other elements. Brass, for example, contains copper and zinc.

Different-sized atoms disturb the regular layers. The layers cannot slide as easily, so an alloy is often harder than the pure metal.

9. Solid, liquid or gas? Follow the particles

The same substance can be a solid, liquid or gas. What changes is the particles’ arrangement and movement.

• Solid: particles are close together and vibrate about fixed positions.
• Liquid: particles stay close together but can move past each other.
• Gas: particles are far apart and move rapidly in all directions.

Heating transfers energy to the substance. During melting or boiling, energy is used to overcome attractions. The particles can then become less ordered or move further apart. Stronger attractions need more energy to overcome, so melting or boiling temperatures are higher.

Cooling reverses these changes: a liquid freezes to form a solid; a gas condenses to form a liquid.

To work out a state, compare the temperature with both the melting point and boiling point. At the stated pressure: below the melting point = solid; between the two points = liquid; above the boiling point = gas. At a change-of-state temperature, two states can be present together while the change happens.

10. A model helps—but it cannot show everything

A diagram makes invisible particles easier to understand. But it is a model, not a photograph. Ask yourself: “What does this show clearly, and what has it left out?”

• Dot-and-cross diagrams: show outer electrons and which atoms supplied them. They usually leave out inner shells, true sizes and the three-dimensional shape. The shell circles in this worksheet are guides for counting electrons, not solid boundaries.
• Ball-and-stick models: show bonds and how atoms are arranged in space. Real atoms are not hard balls joined by sticks. The sizes and gaps may be misleading.
• Flat lattice diagrams: show a small section of a pattern. The real structure extends in three dimensions and contains vastly more particles.
• Space-filling models: help show relative sizes and how particles fit together, but can hide bonds and particles inside the structure.

One copper atom is not “bendy”. Bending and other properties describe how a large arrangement of particles behaves, not one atom on its own.

11. One element, very different materials

Diamond and pencil graphite are both made of carbon. Their very different properties come from the way the carbon atoms are joined.

Diamond: a rigid network. Each carbon makes four strong covalent bonds in three dimensions. There are no layers that slide easily. This makes diamond very hard and useful for cutting tools. Many strong bonds give it a very high melting point. All outer electrons are involved in bonds, so there are no delocalised electrons to carry current.

Graphite: layers that slide. Each carbon makes three covalent bonds in a sheet of hexagonal rings. Strong bonds within the sheets give a high melting point. Weak attractions between sheets let them slide, making graphite useful in pencils and as a lubricant. One electron from each carbon is delocalised. These electrons carry charge, so graphite conducts and can be used as an electrode.

Graphene: just one sheet. Graphene is one layer of graphite, only one atom thick. Strong carbon–carbon bonds make it strong for its mass, and delocalised electrons make it conducting. It can strengthen composite materials (materials made by combining different materials) and can be used in electronic devices.

Fullerenes: hollow carbon molecules. Their structures contain mainly hexagons; pentagons or heptagons can also occur. Buckminsterfullerene, C₆₀, is a roughly spherical cage. Its hollow centre can hold other substances, which is useful in research into drug delivery. The large surface area of fullerenes can also support catalysts.

Carbon nanotubes: hollow cylinders. These cylindrical fullerenes are very long compared with their diameter. Strong bonding and low mass make them useful for reinforcing materials. Suitable nanotubes conduct electricity and can be used in electronics. They are not solid carbon rods.

12. Use the clues to identify a structure

Think like a materials scientist: use more than one clue. A high melting point alone does not tell you whether a substance is ionic, metallic or giant covalent.

To compare electrical conductivity in a school investigation, use a low-voltage circuit and test samples under the same conditions. If a sample dissolves, test its solution in distilled water. Test the distilled water on its own too: this is your control.

Rinse the electrodes between samples so you do not carry substances from one test to the next. Do not try to melt high-melting salts in this classroom comparison.

There is no separate AQA required practical in this topic, but you can still be asked to interpret results and evaluate an investigation.

Identify the structure from evidence

These are invented practice results. Treat each result as a clue. Use at least two clues before choosing a structure.

Practise drawing electron diagrams

Start with H₂ on paper, then try the other seven molecules in section 4. Draw the overlapping outer shells first. Add the shared pairs, then the lone pairs. Check that hydrogen counts two electrons and each other atom counts eight. Next, try the ionic diagrams for MgCl₂ and Na₂O, including brackets and charges. Compare your work with the teaching diagrams before moving on.

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One final thought

Use a scientific idea and a specific example in your reflection.