5: Looking Inside Materials

Question 1

Describe the structure of a metal

Answer 1

Atoms usually form a crystalline lattice where metal atoms are arranged in a regular repeating pattern

Question 2

Why are metals strong and stiff?

Answer 2

Because the electrostatic attraction between the ion lattice and the free electrons forms metallic bonds

Question 3

What makes metals tough?

Answer 3

The strongly bonded lattice structure of a metal

Question 4

Explain why metals are good conductors of heat and electricity

Answer 4

The outer electrons of the metal atoms don’t need much energy to be able to desert their atoms in this crystalline structure. They form a ‘sea’ of free, delocalised electrons, leaving behind a lattice of ions. The free electrons make it a good conductor

Question 5

Why are metals ductile?

Answer 5

The ions within the lattice can move when you apply a force to the metal

Question 6

Describe how atoms move in a metal. Link this to different types of deformation (3)

Answer 6

1) When a force is applied, the interatomic spacing between the ions increases
2) This increase is uniform during elastic deformation
3) Once the stress is high enough to cause plastic deformation, the planes within the metal slip over each other

Question 7

Describe the effect of dislocations in the metal

Answer 7

The stress needed to cause slipping is lower than the stress need to cause slipping in a perfect metal

Question 8

Describe the process of alloying a metal, and describe its effects (3)

Answer 8

1) Atoms of a second metal can be places inside dislocations to pin them down
2) This increases the stress needed to cause slipping
3) Makes the metal harder and less ductile

Question 9

How are ceramics made?

Answer 9

By melting certain materials, and then letting them cool

Question 10

Are ceramics crystalline, polycrystalline or amorphous? What does this mean?

Answer 10

All
Poly: Where there are many regions (grains) of crystalline structure. The atoms in each grain line up in the same direction
Amorphous: There’s no overall pattern. The atoms are arranged at random.

Question 11

How can you change the ceramic making process to increase the chance of a ceramic being amorphous?

Answer 11

The quicker a molten ceramic is cooled, the more likely it is to be amorphous

Question 12

Why do ceramic materials rarely deform plastically before fracture?

Answer 12

The random atomic bonding means that there are no slip planes in ceramic lattices.
They also don’t have mobile dislocations

Question 13

What makes ceramics stiff?

Answer 13

The strong (ionic or covalent) bonds between the atoms

Question 14

What makes ceramics very brittle?

Answer 14

The giant rigid structure

Question 15

What does being brittle mean, for a ceramic when fracturing?

Answer 15

The cracks spread through the ceramic when it fractures.

This is because the applied force acts on a small area (the tip of the crack) so the stress is high

Question 16

What is a polymer?

Answer 16

A long molecular chain, made up of a single repeating unit called a monomer

Question 17

Name a natural and a man-made polymer

Answer 17

Rubber - natural

Polythene - man-made

Question 18

Why are polymers strong?

Answer 18

The monomers in the polymer chain are covalently bonded together, and so are very had to separate

Question 19

Why are polymers flexible?

Answer 19

Polymer chains are often entangled and can be unravelled by rotating about their bonds when you pull them
The more easily the monomers can rotate, the more the chains will untangle, and the more flexible the polymer will be

Question 20

What makes polymers more rigid (less flexible)?

Answer 20

The strength and number of bonds between the chains. The stronger the cross-linking bonds, and the more of them you’ve got, the more rigid the material

Question 21

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

Answer 21

To calculate the size of an atom

Question 22

What was an assumption Rayleigh made?

Answer 22

The oil would spread as much as it could, so this thickness would be the size of one molecule of oil

Question 23

Describe X-ray crystallography

Answer 23

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

Question 24

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

Answer 24

1) Has 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 25

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

Answer 25

0.1-0.5 nm

Question 26

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

Answer 26

Measure atomic sizes
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 27

[3] 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

Answer 27

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)