5: Looking Inside Materials_checked

Question 1

Name a natural and a man-made polymer

Answer 1

Rubber - natural
Polythene - man-made

Question 2

What was the aim of Rayleigh’s oil drop experiment?

Answer 2

To estimate the size of an atom

Question 3

What was an assumption Rayleigh made?

Answer 3

Rayleigh assumed that the oil would spread as much as it could, so this thickness of the oil patch, h, would be the size of one molecule of oil

Question 4

Describe X-ray crystallography

Answer 4

X-ray Crystallography Involves firing X-rays at a sample and using their diffraction patterns to investigate atomic spacing and structure

Question 5

Explain how a scanning tunnelling microscope (STM) works. (5)

Answer 5

1. Scanning Tunneling Microscopes have a very fine tip which a voltage is applied to.
2. Electrons from the sample surface tunnel from the surface to the tip and cause a current to flow
3. The tip is moved across the surface of the sample, and the height of the tip is adjusted to keep the current constant…
   4) ….meaning any small bumps/dips in the surface can be detected
   5) STMs have such a fine resolution that individual atoms can be resolved and their size and spacing measured

Question 6

What do modern techniques say the diameter of an atom is?

Answer 6

0.1 - 0.5 nm depending on the size of the atom

Question 7

What do Scanning Electron Microscopes (SEM) and Atomic Force Microscopes (AFM) do?

Answer 7

Measure atomic sizes
They don’t let you see the surface of the material directly. They build up an atom-by-atom image of the surface on a computer screen. By knowing the magnification of the image on the computer screen and the size of the ‘blobs’ representing each atom, the sizes of atoms can be calculated

Question 8

LiF chips need to be handled with care. Use the internal structure of such ionic crystals to explain how this structure gives these properties
Information given in insert:
Its an ionic lattice.This makes the structure hard but brittle, and easily cleaved

[3]

Answer 8

LiF brittle – so snapping risk/cracks will propagate (1)
due to LiF being an ionic lattice structure/ has
directional bonds (1)
so there is a lack of (mobile) dislocations (1)

Question 9

How did rayleigh carry out his experiment

Answer 9

1. Measure the diameter, d, of the oil drop to calculate the radius, r
2. Place the oil drop in still water and let it spread fully.
3. Measure diameter of patch, D, after it’s spread to calculate radius, R
4. The volume of the oil drop (a sphere) is equal to the volume of the patch of oil (approximate a cylinder).
5. Use equations for Volume and rearrange for h, the thickness of the oil patch which matches size of atom

Question 10

What is the biggest uncertainty in calculating the size of the atom?

Answer 10

1. The largest percentage uncertainty comes from trying to measure diameter of the oil drop of 2 mm to more precise than +-0.5mm, which is a considerable factor
2. In Volume, radius is cubed so this the % uncertainty in radius is a considerable uncertainty

Question 11

What do STM and AFM images show?

Answer 11

STM and AFM images show individual atoms

Question 12

What do SEM images show?

Answer 12

SEM images show larger scale structures than STM and AFM

Question 13

Which has the greater ductility? Pure metals or metal alloys
Why?

Answer 13

1. Metal alloys tend to be less ductile than pure metals
2. This is because metal alloys are formed by the addition of other metals with different sized atoms which can pin dislocations in the structure making slippages more difficult