Hexagonal Close-Packed

Definition

Hexagonal close-packed (HCP) is a type of crystal structure where atoms are arranged in a hexagonal lattice, maximizing space efficiency and achieving a high packing density. This arrangement allows for an efficient way to organize atoms in a solid, contributing to the stability and properties of materials like metals and certain ionic compounds.

5 Must Know Facts For Your Next Test

1. HCP structures have a coordination number of 12, meaning each atom is surrounded by 12 others, optimizing the use of space.
2. This packing arrangement is common in metals such as magnesium, titanium, and zinc, affecting their physical properties like strength and ductility.
3. The hexagonal close-packed structure has an atomic packing factor (APF) of approximately 0.74, indicating a high level of density.
4. HCP can be viewed as two layers of atoms arranged in hexagons, with each additional layer fitting snugly between the previous ones.
5. Unlike cubic structures, HCP has distinct layers that lead to unique slip systems in deformation, impacting how materials behave under stress.

Review Questions

• Compare and contrast the hexagonal close-packed structure with the cubic close-packed structure in terms of packing efficiency and coordination number.
  • Both hexagonal close-packed (HCP) and cubic close-packed (CCP) structures are highly efficient arrangements of atoms with a packing efficiency of about 74%. However, HCP has a coordination number of 12, just like CCP, meaning each atom is surrounded by 12 others. The main difference lies in their geometric arrangement; HCP features alternating layers of atoms that form hexagonal patterns while CCP has a cubic arrangement. This structural difference can lead to variations in the physical properties of the materials that adopt these structures.
• Discuss how the unique layering in hexagonal close-packed structures affects the mechanical properties of metals like titanium and magnesium.
  • The unique layering in hexagonal close-packed (HCP) structures influences the mechanical properties of metals such as titanium and magnesium by determining their slip systems during deformation. In HCP metals, the specific arrangement of atoms results in limited slip systems compared to cubic structures, making them less ductile but often stronger at certain temperatures. This leads to applications where high strength-to-weight ratios are crucial, while also creating challenges during machining processes due to their potential brittleness.
• Evaluate the impact of atomic packing on the conductivity and malleability of materials with hexagonal close-packed structures.
  • The atomic packing in hexagonal close-packed (HCP) structures significantly impacts both conductivity and malleability due to how closely atoms are arranged. The dense packing allows for a higher concentration of free electrons, which enhances electrical conductivity. However, the unique layered structure can restrict atomic movement under stress, affecting malleability. Thus, while HCP materials may offer good conductivity due to their tight atomic arrangements, their ability to deform without breaking is limited compared to cubic structures. This duality plays a crucial role in material selection for various engineering applications.

Related terms

Cubic Close-Packed: A crystal structure similar to HCP, but with a cubic arrangement of atoms that also achieves maximum packing efficiency.

Coordination Number: The number of nearest neighboring atoms surrounding a central atom in a crystal structure, which affects the stability and properties of the solid.

Metallic Bonding: The type of chemical bonding that occurs in metals, characterized by a 'sea of electrons' that allows for conductivity and malleability.