Materials 2: Crystal structures

Question 1

Identify the types of packing that can occur in atomic structures and comment on their energy.

Answer 1

Dense order - Ordered Packing - Lower energies

Non-Dense - Random Packing - Higher energy in comparison to Ordered packing.

Question 2

Define what it means by a crystalline material and identify the types of crystalline structures.

Answer 2

A crystalline material is a structure whose atoms self-organize in a periodic array.

Single Crystal: Atoms are in a repeating or periodic array over the entire extent of the material

Polycrystalline Material: Comprised of many small crystall or grains.

Amorphous Material: The material lacks a systematic atomic arrangement.

Question 3

Give examples of what materials can be defined as Crystalline or Noncrystalline material.

Answer 3

Crystalline: (3D Arrays)

• Metals
• Many Ceramics
• Some polymers

Noncrystalline: (No periodic packing)

• Complex structures
• Rapid cooling

Question 4

Metallic crystal structures tend to be densely packed. give reasons as to why this occurs.

Answer 4

-Typically, only one element is present, so all atomic radii are the same.

-Metallic bonding is not directional (hence, no restriction on numbers or
position of nearest neighbours).

• Nearest neighbour distances tend to be small in order to lower bond energy.
• Electron cloud shields cores from each other.