Question

The co-ordination number of a metal crystallising in a hexagonal close-packed structure is: (a) 12 (b) 4 (c) 8 (d) 6

Short answer

The coordination number of a metal crystallizing in a hexagonal close-packed structure is (a) 12.

Step-by-step solution

Understand the Hexagonal Close-Packed (HCP) Structure

The hexagonal close-packed structure is one of the two simplest types of atomic packing arrangements. In an HCP structure, each sphere (atom) is surrounded by 12 other spheres. This is because the atoms are packed closely together, with each atom touching four atoms in its layer, three in the layer above, and three in the layer below.

Identifying the Coordination Number

The coordination number of an atom in a crystal structure is the number of other atoms to which it is directly connected. For the HCP structure, since each atom is surrounded by 12 other atoms, the coordination number is 12.

Choose the Correct Option

From the given options, we identify that the coordination number of a metal atom in a hexagonal close-packed structure is 12. Therefore, the correct answer is (a) 12.

Key concepts

Hexagonal Close-Packed Structure

Understanding the hexagonal close-packed (HCP) structure is essential when studying materials science and metallurgy. The HCP structure is a highly efficient way of packing atoms together, forming a lattice that is both sturdy and dense. Think of this structure like layers of oranges stacked at a grocery store; each orange resting in the gap formed between others in the layer below.

In HCP, atoms occupy two alternating types of layers, often referred to as A and B. Each layer has its own distinct hexagonal arrangement of atoms and when stacked, they nest into the gaps of the adjacent layers to maximize space efficiency. An atom in one layer is directly above or below the gap in the other layer. This layering process fulfills the principle of highest packing efficiency, by ensuring atoms are as tightly bound and organized as can be.

For a student trying to visualize an HCP structure, it might help to build a model using spheres, such as marbles or balls, to replicate how the atoms are closely packed together. This concrete representation can make the concept of HCP more tangible and easier to understand.

Atomic Packing Arrangements

Delving deeper into atomic packing arrangements, you'll find that the way atoms are organized within a crystal lattice significantly influences the properties of the material. The HCP is just one type of packing arrangement among others, like cubic close-packed (CCP) and body-centered cubic (BCC).

The essence of packing arrangements lies in understanding how each atom interacts with its neighbors. Atoms will arrange themselves in the most stable and energy-efficient configuration possible, and in doing so, they form various geometrical patterns like the hexagon in HCP. These arrangements are not random but based on principles of chemistry and physics that favor stability.

Visualizing Atomic Layers

To aid in the understanding, imagine a tightly packed layer of balls on a flat surface – this would represent a single layer within a crystal lattice. If you then start adding more layers on top, you will naturally seek the most stable configuration for the second layer, which would be nestled into the grooves formed by the first. This is the essence of atomic packing arrangements: finding the pattern that allows the most efficient use of space while maintaining structural integrity.

Crystal Structure Coordination Number

The coordination number is a simple yet powerful concept that tells us how many neighboring atoms (or ions) are immediately adjacent to a chosen atom within a crystal structure. This number can significantly influence the physical properties of the material, including its density and how it interacts with light or electricity.

In the HCP structure, as we've identified, the coordination number is 12. This means that each atom is in direct contact with 12 others, which are distributed as four atoms in the same layer, three in the layer above, and three in the layer below. This distribution ensures the maximum bonding and stability of the structure.

Interpreting Coordination Numbers

The higher the coordination number, the stronger and more tightly bound the structure is likely to be. A student might use this information to predict certain characteristics of materials, such as melting points or mechanical strength. In contrast, materials with a lower coordination number are generally less dense and can have different physical properties. By understanding the coordination number, students can gain insights into why certain materials behave the way they do under various conditions.