Illustrate an octahedral void in the close packing of spheres. Why is it so called ?

Answer

To illustrate an octahedral void in the close packing of spheres, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Close Packing of Spheres**:
- In a close packing arrangement, spheres (representing atoms) are packed together in a way that maximizes their density. The two most common types of close packing are face-centered cubic (FCC) and hexagonal close packing (HCP).

2. **Visualizing the Arrangement**:
- Imagine arranging spheres in layers. Start with a layer of spheres in a triangular arrangement. This layer will be the base layer.

3. **Adding the Second Layer**:
- Place a second layer of spheres above the first layer. The spheres in this layer will fit into the depressions formed by the spheres in the first layer.

4. **Identifying the Octahedral Void**:
- The octahedral void is formed at the center of the arrangement where the spheres from the first and second layers meet. In a simple representation, if you have three spheres forming a triangle in one layer, and three spheres directly above them in the second layer, the space in the center of this arrangement is the octahedral void.

5. **Counting the Surrounding Atoms**:
- An octahedral void is surrounded by six spheres: three above the void and three below it. This gives it a coordination number of six, which is characteristic of octahedral geometry.

6. **Naming the Void**:
- It is called an "octahedral void" because of its geometric shape, which resembles an octahedron. The void is defined by the six surrounding spheres, creating a space that can be visualized as an octahedron.

### Summary:
An octahedral void in the close packing of spheres is formed by the arrangement of spheres where six spheres surround a central void, three above and three below. This is why it is referred to as an octahedral void, reflecting its geometric properties and coordination number.