Sizes of particles and their properties — AQA GCSE Chemistry
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Sizes of particles and their properties explained
Sizes of particles and their properties covers 6 specification points, including Nanoscience refers to structures that are… and Coarse particles are often referred to as dust..
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Nanoscience refers to structures that are 1–100 nm in size, of the order of a few hundred atoms.
Nanoscience studies structures with at least one dimension between 1 nm and 100 nm. A nanometre is 1 × 10⁻⁹ m, so these objects are far smaller than everyday particles but larger than single atoms. Because a typical atom is roughly 0.1–0.3 nm across, a 1–100 nm structure contains of the order of a few hundred atoms, which explains why nanoparticles behave differently from bulk solids. For example, a gold nanoparticle about 10 nm wide contains thousands of atoms arranged in a tiny crystal, yet its surface atoms form a large fraction of the total. Students should be able to compare nanoscale sizes with the diameter of a human hair, about 100 000 nm, and with coarse dust particles measured in micrometres.
Coarse particles are often referred to as dust.
Coarse particles are larger particles, typically with diameters from about 1 × 10⁻⁵ m to 2.5 × 10⁻⁶ m, and they are commonly called dust. They are much bigger than nanoparticles and can be trapped by filters in the nose or in industrial equipment. Their relatively small surface area to volume ratio means only a small proportion of their atoms lie at the surface, so they usually show the familiar bulk properties of the material. Students should distinguish coarse particles from fine particles and nanoparticles by size, and recognise that dust can be produced by grinding, combustion and weathering. Understanding this term helps when comparing the behaviour and health effects of particles of different sizes.
As the side of cube decreases by a factor of 10 the surface area to volume ratio increases by a factor of 10.
For a cube of side length a, surface area is 6a² and volume is a³, so the surface area to volume ratio is 6a² ÷ a³ = 6 ÷ a. If the side length is divided by 10, the ratio becomes 6 ÷ (a ÷ 10) = 60 ÷ a, which is 10 times larger. This inverse relationship explains why smaller particles have a greater proportion of their atoms at the surface. For example, a cube with side 10 cm has a ratio of 0.6 cm⁻¹, while a cube with side 1 cm has a ratio of 6 cm⁻¹. Students should be able to calculate surface area, volume and their ratio, and use the result to predict how properties change with particle size.
Nanoparticles may have properties different from those for the same materials in bulk because of their high surface area to volume ratio.
Nanoparticles have a very large surface area compared with their volume, so a high proportion of their atoms are at the surface. These surface atoms have fewer neighbours and different bonding environments from atoms inside a bulk solid, which can change melting point, reactivity, electrical conductivity, colour and catalytic activity. For example, gold is unreactive and yellow in bulk, but gold nanoparticles can appear red or purple and can act as catalysts. Titanium dioxide nanoparticles are effective in sunscreens because they absorb ultraviolet radiation while being transparent on the skin. Students should link the high surface area to volume ratio to these altered properties rather than treating nanoparticles as simply smaller pieces of bulk material.
It may also mean that smaller quantities are needed to be effective than for materials with normal particle sizes.
Nanoparticles have a very large surface area to volume ratio because a high proportion of their atoms lie at the surface. This means a given mass of nanoparticulate material exposes far more reactive surface than the same mass of a coarse powder, so less material is often needed to achieve the same effect. For example, a thin layer of nano-sized titanium dioxide can give effective sun protection using a fraction of the mass of larger pigment particles, and nano-sized catalysts can speed up a reaction with a smaller mass of catalyst. The statement is a consequence of particle size, not a separate property: reducing particle size increases surface area for the same mass, which can make the material more effective per gram.
Students should be able to compare ‘nano’ dimensions to typical dimensions of atoms and molecules.
Nanoparticles are particles with at least one dimension in the range 1 to 100 nanometres, where 1 nm = 1 × 10⁻⁹ m. Typical atoms are about 0.1 to 0.5 nm across, and small molecules are often around 0.5 to 2 nm across, so a nanoparticle is roughly a few to a few hundred atoms or molecules wide. Fine particles (PM2.5) have diameters between 100 and 2500 nm, while coarse particles (PM10) are larger still, often micrometres across. Nanoparticles sit between individual atoms or molecules and bulk material. Comparing dimensions means using the same unit, usually nanometres, and stating how many atoms or molecules would fit across a nanoparticle.
Your focus
- Define nanoscience as the study of structures measuring 1–100 nm in at least one dimension.
- Convert between nanometres and metres accurately.
- Relate the 1–100 nm size range to a structure containing of the order of a few hundred atoms.
Show all 18 objectives
- Define coarse particles and identify them as dust.
- Compare the size of coarse particles with that of nanoparticles.
- Explain why coarse particles usually display bulk properties rather than nanoscale properties.
- Calculate the surface area to volume ratio of a cube from its side length.
- Demonstrate that reducing the side length by a factor of 10 increases the ratio by a factor of 10.
- Use the ratio to explain why smaller particles have a greater proportion of surface atoms.
- Explain why nanoparticles have a high surface area to volume ratio.
- Describe how a high proportion of surface atoms can change the properties of a material.
- Give examples of properties of nanoparticles that differ from those of the same bulk material.
- Describe why nanoparticulate material can be effective in smaller quantities than material with normal particle sizes.
- Relate the large surface area to volume ratio of nanoparticles to increased contact with surroundings.
- Apply the idea to a named context such as sun cream, medicine or catalysis.
- State the size range of nanoparticles in nanometres and convert between nanometres and metres.
- Compare nanoparticle dimensions with typical sizes of atoms and molecules.
- Place nanoparticles on a size scale between atoms or molecules and fine or coarse particles.
Sizes of particles and their properties exam tips
Marking Points
- States that nanoscience involves structures with at least one dimension from 1 nm to 100 nm.
- Recalls that 1 nm = 1 × 10⁻⁹ m and can convert a nanoscale length into metres.
- Explains that nanoscale structures contain of the order of a few hundred atoms, linking size to atomic scale.
- Compares nanoparticle dimensions with larger particles such as dust or a human hair to place the scale in context.
- Uses the nanometre scale to justify why nanoparticles have a very large surface area to volume ratio.
- States that coarse particles are larger particles commonly known as dust.
- Places coarse particles above the nanoscale, typically in the micrometre range.
- Contrasts coarse particles with nanoparticles in terms of size and surface area to volume ratio.
- Explains that coarse particles generally show bulk properties because most atoms are inside the particle rather than at its surface.
- Links coarse particles to everyday sources such as dust from roads, mining or combustion.
- Recalls that surface area of a cube is 6a² and volume is a³ for side length a.
- Calculates the surface area to volume ratio as 6 ÷ a and recognises its inverse relationship with side length.
- Shows that dividing the side length by 10 multiplies the ratio by 10.
- Applies the relationship to compare cubes of different sizes using correct units.
- Uses the ratio to explain why smaller particles have a higher proportion of surface atoms.
- States that nanoparticles have a high surface area to volume ratio.
- Explains that a high proportion of atoms in a nanoparticle are at the surface.
- Links surface atoms with different bonding or coordination to altered physical and chemical properties.
- Gives a valid example, such as gold nanoparticles having a different colour or catalytic activity from bulk gold.
- Uses the surface area to volume ratio to explain why nanoscale properties differ from bulk properties.
- Nanoparticles have a high proportion of their atoms at the surface, giving a very large surface area to volume ratio.
- For the same mass, nanoparticulate material exposes much more surface area than material with normal particle sizes.
- A larger exposed surface area allows more contact with surrounding reactants or surfaces, so the effect can be achieved with a smaller quantity.
- This can reduce cost, waste and the mass of material needed, which is why smaller quantities may be effective.
- The statement applies to effectiveness per unit mass, not to the intrinsic reactivity of the substance, which is unchanged.
- Nanoparticles have at least one dimension between 1 nm and 100 nm.
- One nanometre is 1 × 10⁻⁹ m, so nano dimensions are much smaller than everyday objects.
- Typical atoms are about 0.1 to 0.5 nm across and small molecules are often about 0.5 to 2 nm across.
- A nanoparticle is therefore roughly a few to a few hundred atoms or molecules across, larger than a single atom or small molecule but smaller than fine and coarse particles.
- Comparisons should use the same unit, such as nanometres, and may use a simple ratio or scale factor.
Examiner Tips
- 💡Quote the range 1–100 nm and define the nanometre in metres when a question asks for the scale of nanoscience.
- 💡Use a familiar comparison, such as a human hair being about 100 000 nm wide, to show the tiny scale.
- 💡When calculating, convert nanometres to metres before substituting into any surface area or volume formula.
- 💡Use the phrase coarse particles are often referred to as dust when defining the term.
- 💡Give a size comparison, such as coarse particles being micrometres across while nanoparticles are 1–100 nm.
- 💡Link the low surface area to volume ratio of coarse particles to their bulk behaviour when explaining properties.
- 💡Write the formulae surface area = 6a² and volume = a³ before substituting values.
- 💡Show the ratio as a fraction and simplify to 6 ÷ a to make the inverse relationship clear.
- 💡Test the factor-of-10 claim with a numerical example, such as side 10 cm and side 1 cm, to confirm the result.
- 💡Always name the property that changes, such as colour, melting point or catalytic activity, and link it to the high surface area to volume ratio.
- 💡Use a named example, such as gold nanoparticles or titanium dioxide in sunscreen, to support the explanation.
- 💡Avoid saying nanoparticles are a new element; emphasise that the bulk material and nanoparticles have the same chemical identity.
- 💡Link the answer explicitly to surface area to volume ratio, then explain how that leads to a smaller effective quantity.
- 💡Use a named context such as sun cream or a catalyst to show the idea applied rather than stated.
- 💡Compare nano-sized particles with normal-sized particles of the same substance in the same sentence to make the contrast clear.
- 💡Quote the 1 to 100 nm range and the value of 1 nm in metres when making a comparison.
- 💡Use a ratio such as a 10 nm particle being about 20 to 100 atoms across, depending on the atom size, to show scale.
- 💡State clearly that nanoparticles are larger than atoms and molecules but smaller than fine and coarse particles.
Common Mistakes
- Treating 1–100 nm as a mass or volume range rather than a length range; correct by specifying that at least one dimension of the structure lies in that range.
- Confusing nanometres with micrometres or millimetres; correct by recalling 1 nm = 1 × 10⁻⁹ m and 1 µm = 1 × 10⁻⁶ m.
- Assuming a nanoparticle contains only a few atoms; correct by explaining that a few hundred atoms is the order of magnitude for structures in this size range.
- Calling nanoparticles dust; correct by reserving the term coarse particles or dust for the larger micrometre-scale particles.
- Assuming dust is a single chemical; correct by describing dust as a mixture of particles of various materials and sizes.
- Believing coarse particles have a high surface area to volume ratio; correct by stating that their ratio is low compared with nanoparticles.
- Squaring the side length when calculating volume; correct by using a³ for volume and 6a² for surface area.
- Thinking the ratio increases by a factor of 100 when the side decreases by 10; correct by showing the ratio is inversely proportional to side length, so the factor is 10.
- Omitting units or using inconsistent units; correct by keeping all lengths in the same unit and expressing the ratio with a unit such as cm⁻¹.
- Claiming nanoparticles are a different chemical element from the bulk material; correct by stating they are the same material with different particle size and surface effects.
- Saying nanoparticles have a high surface area to volume ratio because they are small, without explaining that a greater fraction of atoms are at the surface; correct by explicitly linking the ratio to surface atom proportion.
- Assuming all properties change in the same way for every material; correct by giving specific examples and noting that the effect depends on the material and property.
- Thinking nanoparticles are a different chemical substance with new chemical properties; the correction is that the substance is the same and the change is in surface area to volume ratio and related behaviour.
- Assuming smaller particles always mean a faster or stronger effect in every context; the correction is that the effect depends on the application, such as catalysis, sun cream or medicine.
- Confusing surface area with total volume; the correction is that for the same mass the volume is unchanged but the exposed surface area increases as particle size decreases.
- Treating a nanoparticle as a single atom or molecule; the correction is that a nanoparticle contains many atoms or molecules and is larger than both.
- Mixing up nanometres and metres without converting; the correction is to convert using 1 nm = 1 × 10⁻⁹ m before comparing.
- Describing nanoparticles as simply very small without giving a scale; the correction is to quote the 1 to 100 nm range and compare it with atomic or molecular sizes.