Looking inside materials

Question 1

Estimating the size of atoms and molecules

Rayleigh’s oil drop experiment

Answer 1

Volume of oil drop = 4/3 π r^3
Volume of oil patch = π R^2 h

Oil drop = oil patch

h = (4r^3) / (3R^2)

h = diameter of an oil 
r = radius of oil drop (contains the greatest uncertainty)
R = radius of oil patch

Question 2

Estimating the size of atoms and molecules

Using density

Answer 2

You need:

• mass of a particular atom
• density of the element

Density = mass / volume

Work out the mass of 1m^3 of the element
Work out the number of atoms in 1m^3 by dividing this mass by the mass of one atom
Work out the volume of one atom and therefore an estimate for its diameter

Question 3

Modern methods for measuring atoms

Answer 3

Individual atoms:
AFM: atomic force microscope
STM: Scanning tunneling microscope

Larger scale structures:
SEM: Scanning electron microscope

Question 4

Metallic structure

Answer 4

Crystalline, regular lattice structure of positive metal ions and delocalised electrons
Bonds are ionic and non directional
Mobile dislocations allow layers to slide over one another while requiring relatively low amounts of energy
Alloys have less mobile dislocations as alloy atoms pin the dislocations

Question 5

Ceramic structure

Answer 5

Achieved through heating and cooling in a furnace
Compounds between metallic and non-metallic elements
The bonds are either totally ionic or predominantly ionic with some covalent
They are also directional, meaning the atoms cannot slip, which is why ceramics are hard and brittle
Either crystalline or semi crystalline

Question 6

Polymer structure

Answer 6

Long chains of molecules of strong covalent bonds that are hard to break but can rotate easily
This makes polymers strong and flexible
However if there are cross links it is more difficult for the bonds to rotate; polymers with cross links are stiffer
For example, rubber is a runny white liquid of polymer chains, until you add sulphur (vulcanisation), which forms cross links making rubber stiff enough for elastic bands and tyres.
More sulphur = more cross links = stiffer

Question 7

Polymer structure

Answer 7

Long chains of molecules of strong covalent bonds that are hard to break but can rotate easily
This makes polymers strong and flexible
However if there are cross links it is more difficult for the bonds to rotate; polymers with cross links are stiffer
For example, rubber is a runny white liquid of polymer chains, until you add sulphur (vulcanisation), which forms cross links making rubber stiff enough for elastic bands and tyres.
More sulphur = more cross links = stiffer

Question 8

Metal elastic extensibility

Answer 8

0.1%

Question 9

Polymer elastic extensibility

Answer 9

1%

Question 10

Polymer plastic extensibility

Answer 10

> 100%

When polymers are stretched plastically, more of the chains line up and it becomes more crystalline