Hexagonal Close-Packed

Definition

Hexagonal close-packed (hcp) is a type of crystal structure where atoms are arranged in a hexagonal lattice, maximizing the density of atoms in a given volume. This arrangement is characterized by a coordination number of 12, meaning each atom is in contact with 12 others, and it features alternating layers that contribute to its efficient packing. The hcp structure is commonly found in metals and plays a crucial role in understanding solid state structures.

5 Must Know Facts For Your Next Test

1. The hexagonal close-packed structure consists of two alternating layers, with one layer directly above another, maximizing space utilization.
2. Hcp has a packing efficiency of approximately 74%, meaning that 74% of the volume is occupied by atoms.
3. Common metals that crystallize in the hcp structure include magnesium, titanium, and zinc.
4. In hcp, the atoms are arranged such that there are two different types of positions: 'A' and 'B', leading to the ABAB stacking sequence.
5. Understanding hcp is essential for predicting the properties of materials, as this structure influences factors such as ductility and hardness.

Review Questions

• How does the hexagonal close-packed structure compare to other close-packed structures in terms of atomic arrangement and packing efficiency?
  • The hexagonal close-packed structure features an arrangement where atoms are organized into two alternating layers, resulting in a specific ABAB stacking sequence. In comparison to cubic close-packed structures, which also achieve high packing efficiency but through a different arrangement, hcp has a similar packing efficiency of about 74%. The differences in atomic arrangement between these structures can influence various material properties such as strength and ductility.
• Discuss the significance of coordination number in the context of the hexagonal close-packed structure and its implications for material properties.
  • In the hexagonal close-packed structure, the coordination number is 12, indicating that each atom touches 12 neighboring atoms. This high coordination number contributes to the stability and strength of materials exhibiting this structure. A higher coordination number often leads to increased metallic character and influences how the material behaves under stress, affecting properties like ductility and malleability.
• Evaluate how understanding the hexagonal close-packed structure can impact advancements in materials science and engineering.
  • Grasping the details of the hexagonal close-packed structure allows materials scientists to tailor materials for specific applications. For instance, knowing how this arrangement influences mechanical properties enables engineers to design stronger and more durable materials for aerospace or automotive industries. Furthermore, innovations in alloy design can emerge from understanding how different elements might fit into an hcp lattice, leading to advancements in performance characteristics such as lightweight structures or enhanced thermal stability.

Related terms

Cubic Close-Packed: A crystal structure similar to hcp but with atoms arranged in a cubic formation, also achieving high packing efficiency.

Coordination Number:

The number of nearest neighboring atoms surrounding a central atom in a crystal structure.

Unit Cell: The smallest repeating unit in a crystal lattice that defines the entire structure and its properties.