Hexagonal Close Packing

Definition

Hexagonal close packing (HCP) is a specific arrangement of spheres where each sphere is surrounded by twelve others in a compact and efficient manner. This packing structure is significant because it maximizes the density of spheres within a given volume, achieving about 74% packing efficiency. HCP is commonly found in various crystal structures, making it essential for understanding material properties and behaviors in different contexts.

5 Must Know Facts For Your Next Test

1. In hexagonal close packing, spheres are stacked in layers with an ABAB pattern, where 'A' and 'B' denote two different types of layers.
2. The first layer of spheres is arranged in a hexagonal shape, while the second layer fits into the gaps of the first, enhancing space utilization.
3. HCP has a coordination number of 12, meaning each sphere touches 12 others, providing stability and strength to the structure.
4. This packing arrangement is often found in metals like magnesium and titanium, influencing their mechanical properties.
5. Hexagonal close packing can also influence phase transitions in materials, affecting how substances behave under different temperature and pressure conditions.

Review Questions

• How does the stacking arrangement in hexagonal close packing contribute to its efficiency?
  • The stacking arrangement in hexagonal close packing follows an ABAB pattern, which allows each sphere to be tightly surrounded by twelve others. This optimal arrangement maximizes the density of packed spheres, achieving about 74% packing efficiency. The way layers fit into each other minimizes empty space, making HCP one of the most efficient ways to arrange spheres.
• Compare and contrast hexagonal close packing with face-centered cubic packing regarding their structural characteristics.
  • Both hexagonal close packing (HCP) and face-centered cubic (FCC) arrangements achieve high packing efficiency of around 74%. However, HCP features an ABAB stacking pattern with two distinct layers, while FCC has a three-layer ABCABC stacking sequence. This difference in stacking leads to variations in coordination numbers and interstitial sites available for atoms or ions within these structures.
• Evaluate the impact of hexagonal close packing on the properties of materials such as metals and how it influences their applications.
  • Hexagonal close packing significantly affects the mechanical properties of metals that adopt this structure, such as magnesium and titanium. The strong bonding interactions due to the high coordination number of 12 contribute to these metals' hardness and ductility. Understanding HCP allows engineers and material scientists to tailor materials for specific applications by predicting how they will behave under stress, temperature changes, or phase transitions, ultimately informing decisions in industries like aerospace and automotive engineering.

Related terms

Packing Efficiency: The ratio of the volume occupied by the spheres to the total volume of the container in which the spheres are packed.

Face-Centered Cubic: A type of crystal lattice structure where atoms are located at each corner and the centers of all the cube faces, resulting in high packing efficiency similar to HCP.

Close Packing: A general term for arrangements of spheres that allow for maximum density, including both hexagonal close packing and face-centered cubic arrangements.