Question

What is the co-ordination number of hcp crystal lattice?

• A. 8
• B. 12
• C. 6
• D. 6

Answer 1

The Correct Option is B

Concept:
The coordination number in crystallography refers to the number of nearest neighboring atoms or ions that are directly in contact with a given atom or ion in a crystal structure. It is a crucial concept that determines how atoms are arranged and how tightly they are packed in a solid.

Hexagonal Close-Packed (hcp) Structure:
In a hexagonal close-packed (hcp) crystal lattice, atoms are arranged in a compact and efficient three-layered structure. The stacking follows an ABAB pattern, where:
- The first layer (A) is placed at the base.
- The second layer (B) is placed in the depressions of the first layer.
- The third layer is aligned exactly above the first layer (A).

This arrangement allows the atoms to occupy maximum space efficiently.

Coordination Number in hcp:
In an hcp lattice:
- Each atom is in contact with 6 atoms in its own layer.
- It also touches 3 atoms in the layer above.
- And 3 atoms in the layer below.

This gives a total of 12 nearest neighbors, which means the coordination number is 12. This high coordination number is responsible for the excellent packing efficiency of about 74 percent, making hcp one of the most efficient packing structures in solids.

Examples of Elements with hcp Structure:
- Magnesium (Mg)
- Zinc (Zn)
- Titanium (Ti)

Conclusion:
The hexagonal close-packed (hcp) crystal structure has a coordination number of 12. Each atom is closely surrounded by 12 other atoms, resulting in a tightly packed, stable structure.

Therefore, the correct option is: (B) 12.