Simple and giant structures booklet
Simple and giant structures booklet
- Unit 1: Particles & the Nature of Matter
- Unit 2: Elements, Compounds & Mixtures
- Unit 3: Properties of Materials
- Unit 4: Acids, Alkalis & Indicators
- Unit 5: Chemical Reactions
- Unit 6: Reactivity & Making Salts
- Unit 7: Rates of Reaction
In this topic you will:
- learn how giant structures are formed
- compare the properties of ionic and covalent substances
- explain how the structures of these substances relate to their properties
Key words
- graphite
- intermolecular forces
- lattice
- layers
- macromolecule
Giant structures in ionic compounds
Sodium chloride is an ionic compound. The ions of sodium, Na+, and chlorine, Cl−, have equal and opposite electrical charges so they are strongly attracted to one another. These forces, called electrostatic forces, act in all directions and form ionic bonds.
The ions in sodium chloride make a giant structure known as a lattice.
In the lattice structure, a sodium ion, Na+, is surrounded by six Cl− ions. Sodium chloride forms crystals with a regular shape because the ions are arranged in a regular pattern.
Giant covalent structures
Many substances that have covalent bonds are formed of simple molecules: oxygen, carbon dioxide and methane, for example. This is because the forces holding the molecules together are very strong but the forces between the molecules are weak. The forces between the molecules are called intermolecular forces.
However, some covalent substances such as silicon dioxide have giant covalent structures.
Giant structures of carbon
The carbon atoms in diamond form a giant structure. Each carbon atom forms four strong covalent bonds.
Diamond is the hardest material on Earth. It is not just used for jewellery but it is used for cutting and drilling tools. It is so hard because of the strong, rigid, three-dimensional structure of the lattice.
These large structures are called macromolecules.
Carbon also forms a giant structure for a very soft material, graphite.
Graphite is used for the ‘lead’ in pencils, and for lubricating moving parts in machines.
In graphite, the carbon atoms each make bonds with three other atoms. This forms layers, which can easily slide over one another. The covalent bonds between the carbon atoms in the layers are strong. The bonds between the layers are weak so the layers slide over each other very easily. This makes the surface very soft and it easily comes away. This is what happens when you make a pencil mark on paper.
Think about it
Questions
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In an ionic bond, electrons are transferred from one atom to another, usually between a metal and a non-metal. In a covalent bond, electrons are shared between non-metal atoms.
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An ionic compound is formed when a metal reacts with a non-metal.
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A molecule is a group of atoms bonded together. In covalent molecules, the atoms are held by shared pairs of electrons.
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A macromolecule is a very large molecule made of many atoms joined together in a giant structure. Diamond is an example of a macromolecule made of carbon atoms.
Covalent and ionic substances have different properties
Melting and boiling points
Ionic substances have very high melting points and boiling points. This is because there are very strong electrostatic forces holding the ions together.
Covalent substances made from simple molecules have low melting points and boiling points because, although the forces holding the molecules together are strong, the forces between the molecules – intermolecular forces – are weak. This means that only a small amount of energy is needed to overcome these forces in order to melt or boil them.
Questions
Use this table to help answer questions 5–9.
| Substance | Melting point in °C | Boiling point in °C |
|---|---|---|
| sodium chloride | 801 | 1413 |
| methane | –182 | –161 |
| magnesium chloride | 714 | 1412 |
| ammonia | –77 | –34 |
| calcium oxide | 2613 | 2850 |
| chlorine | –101 | –34.6 |
| water | 0 | 100 |
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Magnesium chloride is an ionic compound. It has a high melting point, which indicates strong electrostatic forces between ions.
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Ammonia is a covalent molecule. Its very low melting and boiling points suggest weak intermolecular forces between simple molecules.
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Ammonia is a gas at room temperature, since its boiling point is –34 °C.
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They are ionic compounds with strong electrostatic forces between oppositely charged ions that require lots of energy to overcome.
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They are simple covalent molecules with weak intermolecular forces, so only a little energy is needed to separate them.
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Copper sulfate is an ionic compound. The crystal shape and solid form suggest a regular ionic lattice of copper and sulfate ions.
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The very high melting and boiling points suggest giant covalent bonding, with strong bonds throughout the entire structure.
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Its hardness and very high melting point suggest strong covalent bonds throughout the structure, indicating a giant covalent lattice.
Conducting electricity
Ionic compounds will conduct electricity if they are dissolved in water or if they are melted to form a liquid.
Ionic compounds can conduct electricity because the ions have an electrical charge. The ions must be free to move about and carry the electrical charge.
Covalent substances made from simple molecules do not conduct electricity.
Think Like a Scientist
Investigation: Do Ionic Compounds Conduct Electricity?
In this investigation, you will test both ionic solutions and solid crystals to find out if they conduct electricity. You will observe how conductivity depends on the physical state of the compound.
You will need: safety glasses, surgical gloves, electrical wires, lamp, cell, carbon electrodes, beaker, crystals of ionic compounds (e.g. copper sulfate, sodium chloride), water
Safety: Some ionic compounds like copper sulfate are irritants. Avoid touching them directly. Always wear safety glasses and gloves while handling solids or solutions. Dispose of chemicals properly and avoid skin contact.
Steps:
1. Set up the circuit using a bulb and battery, but without connecting any substance yet. Make sure the bulb lights up when tested.
2. Place a solution of an ionic compound in a beaker.
3. Connect the beaker into the circuit using carbon electrodes (as shown in the diagram).
4. Complete the circuit and observe whether the bulb lights up.
5. Repeat this process for all the ionic solutions you are testing.
6. Record which ones conduct electricity.
7. Now test the solid (crystal) forms of each compound. Insert a dry crystal directly into the circuit (as shown).
8. Observe if the bulb lights up and record your results.
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Yes, all the ionic solutions conducted electricity because their ions were free to move in water.
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In solution, ionic compounds break apart into positive and negative ions. These moving ions carry electric current through the liquid.
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No, the solid crystals did not conduct electricity.
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In solid form, ions are fixed in place and cannot move. Without free-moving charged particles, electricity cannot flow.
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Covalent substances don’t form ions, so they would not conduct electricity. Their molecules are neutral and don't carry current.
Summary of the properties of ionic and covalent substances
| Ionic substances | Covalent substances |
|---|---|
| Compounds made from ions form giant lattices. These ionic compounds have very high melting points and boiling points because the forces between the ions are so strong. | Substances made from simple molecules have low melting points and boiling points because there are only weak forces between the molecules. Atoms that share electrons can form giant structures called macromolecules. These have very high melting points because the atoms are joined by strong covalent bonds. |
| When ionic compounds are dissolved in water or melted, they can conduct electricity because the ions are free to move about and carry the electrical charge. | Simple covalent molecules do not conduct electricity. |