Structures, Properties and Nanoparticles
Properties follow from structure, and GCSE chemistry covers four structures: giant ionic lattices, which have high melting points and conduct only when molten or dissolved; simple molecular substances, with low melting points because only the weak intermolecular forces between molecules are overcome on melting; giant covalent structures such as diamond and graphite, with very high melting points because every atom is joined by strong covalent bonds; and polymers, large molecules held together by strong covalent bonds along the chain.
Before you start
Make sure you're comfortable with these topics first:
Method
- Identify the structure first, then answer the property question from it. Almost every mark scheme in this topic follows the pattern: name the structure, name the force that must be overcome, comment on whether charged particles are free to move.
- For melting and boiling points, say precisely what is broken: in simple molecular substances the weak intermolecular forces are overcome and the covalent bonds are not, which is the single most commonly missed mark in the topic.
- For electrical conductivity, apply one rule: a substance conducts if it contains charged particles that are free to move. That means delocalised electrons (metals, graphite) or free-moving ions (molten or dissolved ionic compounds).
- Learn diamond and graphite as a contrast: in diamond each carbon forms four covalent bonds in a rigid three-dimensional structure, so it is very hard and does not conduct; in graphite each carbon forms three bonds in layers of hexagonal rings with weak forces between layers, so layers slide (making it soft and slippery) and the fourth outer electron per atom is delocalised, so it conducts.
- Learn graphene and the fullerenes briefly with a use each: graphene is a single layer of graphite, strong and conducting, used in electronics and composites; buckminsterfullerene C60 is a hollow sphere; carbon nanotubes have a very high length-to-diameter ratio and are used to reinforce materials and in electronics.
- For nanoparticle questions, always start from the surface area to volume ratio: as the side length falls by a factor of 10, the ratio increases by a factor of 10, so far less material achieves the same effect. Then balance a use against the risk that the long-term effects on health of particles small enough to enter cells are not fully known.
Worked example
A cube of side 100 nm is divided into cubes of side 10 nm. Compare the surface area to volume ratio of one large cube with that of one small cube, and explain why this matters for catalysts.
- Large cube: surface area = 6 x 100 x 100 = 60 000 square nanometres; volume = 100 x 100 x 100 = 1 000 000 cubic nanometres.
- Ratio for the large cube: 60 000 / 1 000 000 = 0.06 to 1.
- Small cube: surface area = 6 x 10 x 10 = 600 square nanometres; volume = 10 x 10 x 10 = 1000 cubic nanometres.
- Ratio for the small cube: 600 / 1000 = 0.6 to 1.
- Compare: the ratio is 10 times greater for the smaller cube, matching the rule that dividing the side length by 10 multiplies the surface area to volume ratio by 10.
- Apply it: catalysis happens on the surface, so a nanoparticulate catalyst exposes far more surface per gram, and a much smaller mass of an expensive metal achieves the same rate.
Practice questions
Try each question, then tap to reveal the answer.
Q1Why does simple molecular carbon dioxide have a much lower melting point than giant covalent silicon dioxide?Show answer
Answer: Melting carbon dioxide only overcomes weak intermolecular forces between molecules, while melting silicon dioxide requires breaking many strong covalent bonds throughout the giant structure.
Q2Explain why graphite conducts electricity but diamond does not.Show answer
Answer: In graphite each carbon atom forms three covalent bonds, leaving one delocalised electron per atom that is free to move; in diamond all four outer electrons are used in covalent bonds, so none are free.
Q3Why is graphite soft and slippery?Show answer
Answer: It is made of layers held together by weak intermolecular forces, so the layers can slide over each other.
Q4State the size range of a nanoparticle.Show answer
Answer: Between 1 and 100 nanometres across.
Q5Give one use of nanoparticles and one associated risk.Show answer
Answer: Use: nanoparticulate titanium dioxide in sun cream gives better UV protection without leaving a white residue. Risk: the long-term effects on health of particles small enough to be absorbed into cells are not fully known.
Q6Why do polymers have higher melting points than small molecules such as methane?Show answer
Answer: Polymer molecules are very large, so the total intermolecular forces between chains are much greater and more energy is needed to separate them.
Q7Why can an ionic solid not conduct electricity?Show answer
Answer: Its ions are held in fixed positions in the lattice, so although they are charged they are not free to move.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
Diamond and graphite are both made only of carbon atoms, but diamond is used in cutting tools and graphite is used in pencils and as a lubricant. Explain these uses in terms of structure and bonding.
Show mark scheme
Tick each line you got. Your score builds from the marks on the scheme.
Nothing ticked yet - 6 available
Explain why sodium chloride conducts electricity when dissolved in water but not as a solid.
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Nothing ticked yet - 3 available
See real GCSE Science past-paper questions, with official mark schemes →
Free printable worksheet
Want more practice on paper? Download the structures, properties and nanoparticles worksheet pack - 17 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 19 of GCSE Chemistry Workbook, the whole course as one free printable PDF.
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