AQA GCSE Chemistry: Bonding, structure, and the properties of matter 6-mark questions

This method is not reliable because physical state alone does not identify the type of bonding. A solid could be ionic, metallic or covalent, and a gas could contain covalent molecules. The physical state depends on the structure of the particles and the strength of the attractions between them. For example, magnesium oxide is a solid, so step 2 would classify it as ionic, but this is only correct because of its elements and structure. Magnesium is a metal and oxygen is a non-metal, so electrons are transferred from magnesium atoms to oxygen atoms, forming positive magnesium ions and negative oxide ions. These ions are arranged in a regular giant ionic lattice, with strong electrostatic attractions between oppositely charged ions in all directions. Carbon dioxide is a gas and contains only non-metals, carbon and oxygen, so it has covalent bonding. The atoms share pairs of electrons to form small, separate carbon dioxide molecules. There are only weak intermolecular attractions between these molecules, which helps explain why carbon dioxide is a gas. However, being a gas is not by itself proof that a substance is covalent. The rules are also inconsistent for copper. Copper is a solid, so step 2 would classify it as ionic, but it conducts electricity as a solid, so step 4 would classify it as metallic. Copper contains metal atoms and has metallic bonding: positive metal ions are arranged in a giant structure and are attracted to delocalised electrons. These electrons can move through the structure, allowing solid copper to conduct electricity. In contrast, ions in a solid ionic lattice cannot move freely. The method should first identify the elements present. A metal combined with a non-metal suggests ionic bonding, non-metals only suggests covalent bonding, and metal atoms suggest metallic bonding. This should then be checked using the particle arrangement: a giant lattice of oppositely charged ions indicates ionic bonding, small molecules formed by shared electrons indicate covalent bonding, and a giant structure of metal ions with delocalised electrons indicates metallic bonding.

In ice, the water particles are close together in a fixed, regular arrangement and can only vibrate about fixed positions. As the ice is heated to its melting point, the particles gain kinetic energy and vibrate more, but during melting the energy supplied is used to overcome some of the forces of attraction holding them in this arrangement. Their average kinetic energy therefore does not increase, so the temperature remains at 0 °C. In liquid water, the particles are still close together but are no longer fixed in position, so they can move and slide past one another. Once all the ice has melted, further heating increases the particles’ average kinetic energy, so they move faster and the temperature rises. During boiling, the energy supplied is used to overcome the remaining forces of attraction between the particles, allowing them to separate and form a gas. The average kinetic energy does not increase during boiling, so the temperature remains at 100 °C. In water vapour, the particles are far apart and move rapidly in random directions. After all the water has boiled, further energy increases their average kinetic energy and the temperature rises again.

The student is incorrect because graphene is not graphite broken into smaller pieces. Graphene is one single layer of carbon atoms, one atom thick, arranged in hexagonal rings, whereas graphite consists of many of these layers held together by weak forces. The layers in graphite can slide over each other, so graphite is soft and slippery. In graphene, each carbon atom forms three covalent bonds, leaving one delocalised electron that can move through the layer. This means graphene conducts electricity well. It is also very strong but has an extremely low mass, so it can make an electrical composite both conductive and strong without adding much weight. Carbon nanotubes are different again: they are cylindrical fullerenes made from carbon atoms arranged in tubes. Their very high length-to-diameter ratio and strong carbon bonds allow them to reinforce other materials, so they are useful for making tennis rackets stronger without making them excessively heavy.

A nanoparticle has a diameter of about 1–100 nm and consists of a few hundred atoms, so it is not a single atom. Nanoparticles have a much greater surface area to volume ratio than the same material in bulk form. This can give them different properties and means that a smaller amount may be needed to produce the same effect. Their very small size and different properties make them useful in medicine: cage-like nanoparticles can contain a medicine and deliver it directly to tumour cells, rather than using a large bulk sample that could affect other cells. The student has confused a nanoparticle with an individual atom; a nanoparticle is a structure made from many atoms, so its properties are not necessarily the same as the properties of an individual atom.