GCSE · Chemistry · AQA · Spec 8462

Sizes of particles and nanoparticles

Grind a substance finer and finer. Nothing about the substance changes, only the size. Yet at the nanoscale, its pieces can behave differently from the lump they came from.

From atoms to dust

Tap each particle from left to right. 1 nm (one nanometre) = 1 × 10⁻⁹ m.

10⁻¹⁰ m10⁻⁹ m10⁻⁸ m10⁻⁷ m10⁻⁶ m10⁻⁵ mSize across, in metres (each tick is ×10)

Atom

radius ≈ 0.1 nm

An atom has a radius of about 0.1 nm (1 × 10⁻¹⁰ m), so it is roughly 0.2 nm across. This is the building block every other particle on this axis is made from.

Surface area to volume ratio

Shrink the cube, work out the ratio

Cube A has sides 100 nm long. Cube B has sides 10 nm long. Work out the surface area to volume ratio (SA : V) of each cube, then compare them.

  1. Cube A: surface area = 6 faces × (100 × 100) = 60 000 nm²A cube has six square faces, and each face has area side × side.
  2. missing step
Which line is step 2?

Why size changes properties

?

Reason it through

Why can nanoparticles behave differently from ordinary lumps of the same substance?

Link 1 of 4

First link · your turn

A particle is made much, much smaller. What happens to its surface area to volume ratio?

2
Locked — reveal the link above first
3
Locked — reveal the link above first
4
Locked — reveal the link above first

What do you think?

Same substance, just smaller?

A substance is well known to be harmless as an ordinary powder. A company starts making nanoparticles of exactly the same substance.

Which is closest to what you think right now?
How sure are you?

Evaluate one use

Should sun creams contain nanoparticles?

The claim

Nanoparticles should be used in sun creams.

Place each piece of evidence to load the balance. Mark the strong ones — they count double.

  1. Sun creams containing nanoparticles give good protection from the sun's ultraviolet (UV) light.

    Evidence 1: does it support or challenge the claim?

  2. Because of their high surface area to volume ratio, only a small quantity of nanoparticles is needed to be effective.

    Evidence 2: does it support or challenge the claim?

  3. Because of their high surface area to volume ratio, nanoparticles may have properties different from the same material in bulk.

    Evidence 3: does it support or challenge the claim?

  4. There are possible risks associated with the use of nanoparticles.

    Evidence 4: does it support or challenge the claim?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

How small a nanoparticle really is, and why shrinking a particle can change how a substance behaves.

What you need to know

  • Nanoscience is about structures 1–100 nm (1 × 10⁻⁹ m to 1 × 10⁻⁷ m) in size, of the order of a few hundred atoms.
  • Fine particles (PM2.5) are 100–2500 nm (1 × 10⁻⁷ m to 2.5 × 10⁻⁶ m) across. Coarse particles (PM10), often called dust, are 2.5 × 10⁻⁶ m to 1 × 10⁻⁵ m across.
  • An atom has a radius of about 0.1 nm, so nanoparticles are larger than atoms and small molecules but far smaller than dust.
  • When a cube's side decreases by a factor of 10, its surface area to volume ratio increases by a factor of 10.
  • Because of their high surface area to volume ratio, nanoparticles may have properties different from the same material in bulk, and smaller quantities may be needed to be effective.
  • Nanoparticles are used in medicine, electronics, cosmetics and sun creams, deodorants and catalysts. There are possible risks, so each use should be evaluated by weighing advantages against disadvantages.

The big picture

Nanoparticles are 1–100 nm across, far smaller than fine particles (PM2.5, 100–2500 nm) and coarse particles (PM10, or dust), and even the largest is only a few hundred atoms across. Making a particle smaller raises its surface area to volume ratio: divide a cube's side by 10 and the ratio is multiplied by 10. So nanoparticles can have different properties from the same material in bulk, and smaller quantities may be effective. That makes them useful in medicine, electronics, cosmetics and sun creams, deodorants and catalysts, but they carry possible risks, so each use has to be evaluated.

Key points

1Nanoparticles: 1–100 nm. Fine particles (PM2.5): 100–2500 nm. Coarse particles (PM10, dust): 2.5 × 10⁻⁶ m to 1 × 10⁻⁵ m.
2From an atom to the largest dust particle is a factor of about 50 000, nearly five powers of ten, so a log scale (each step ×10) is needed to show every class together.
3Surface area to volume ratio = surface area ÷ volume. For a cube, surface area = 6 × side² and volume = side³.
4Divide the side by 10 and the surface area to volume ratio is multiplied by 10.
5A very high surface area to volume ratio puts much more of the material at the surface, so nanoparticles can behave differently from the bulk material and may work in smaller quantities.
6Nanoparticles have possible risks, so a use of nanoparticles is evaluated by weighing its benefits against those risks and reaching a judgement.

Worked example

Problem

A 1 cm cube of a solid catalyst is cut up into small cubes with sides of 1 mm. Nothing is lost, so the total volume stays 1 cm³. How does the surface area to volume ratio change?

⚠ Watch out

Thinking a smaller particle has a smaller surface area to volume ratio. Each particle's surface area does shrink, but its volume shrinks faster, so the ratio goes UP. Divide the side by 10 and SA : V is multiplied by 10, not divided by 10.

🧠

Memory hook

1, 100, 2500, 10 000 nm: nano, then fine, then coarse. And for the ratio: shrink the side ten times, grow SA : V ten times.

✓

Check yourself

Without a calculator: a cube's sides become 100 times shorter. What happens to its surface area to volume ratio, and why could particles that small behave differently?

Flashcards

(12)
What size range counts as the nanoscale?
1–100 nm, which is 1 × 10⁻⁹ m to 1 × 10⁻⁷ m.
What is 1 nanometre (1 nm) in metres?
1 × 10⁻⁹ m, a billionth of a metre.
What diameters do fine particles (PM2.5) have?
Between 100 and 2500 nm (1 × 10⁻⁷ m and 2.5 × 10⁻⁶ m).
What diameters do coarse particles (PM10) have, and what are they often called?
Between 2.5 × 10⁻⁶ m and 1 × 10⁻⁵ m. They are often called dust.
How does the size of a nanoparticle compare with the size of an atom?
An atom has a radius of about 0.1 nm. A nanoparticle is larger, but even a 100 nm one is only a few hundred atoms across.
How do you find the surface area to volume ratio of a cube?
Surface area = 6 × side². Volume = side³. Divide surface area by volume, using the same length unit for both.
A cube's side decreases by a factor of 10. What happens to its surface area to volume ratio?
It increases by a factor of 10.
Why may nanoparticles have different properties from the same material in bulk?
Their very high surface area to volume ratio means much more of the material is at the surface.
Why may smaller quantities of nanoparticles be needed to be effective?
The same mass of nanoparticles exposes far more surface than normal-sized particles, so less material may do the same job.
Name five areas where nanoparticles are used.
Medicine, electronics, cosmetics and sun creams, deodorants, and catalysts.
Why are there possible risks with nanoparticles?
Because of their high surface area to volume ratio, they may have properties different from the same material in bulk.
How do you evaluate a specified use of nanoparticles?
Weigh its advantages against its disadvantages and possible risks, then reach a judgement and give your reason.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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