Coordination number in ABAB … type arrangement is

Options

A. 6
B. 8
C. 12
D. 4

Answer

To determine the coordination number in an ABAB type arrangement, we follow these steps:

### Step-by-Step Solution:

1. **Understanding the ABAB Arrangement**:
- The ABAB arrangement refers to a type of close packing where two different types of atoms (A and B) alternate in layers. This is characteristic of Hexagonal Close Packing (HCP).

2. **Visualizing the Layers**:
- In the first layer (Layer A), we have atoms arranged in a hexagonal pattern. Each atom in this layer is surrounded by 6 other atoms.
- The second layer (Layer B) is placed above the first layer, filling the gaps created by the first layer.

3. **Counting Neighbors**:
- Each atom in Layer B will also be surrounded by atoms from Layer A and Layer B.
- An atom in Layer B touches 3 atoms directly above it in Layer A and 3 atoms directly below it in Layer A.

4. **Calculating Coordination Number**:
- For an atom in Layer A:
- It is surrounded by 6 atoms from Layer B (in the same layer) and 3 atoms from the layer above and 3 atoms from the layer below.
- Therefore, the total coordination number for an atom in Layer A is:
\[ 6 \text{ (from Layer B)} + 3 \text{ (above)} + 3 \text{ (below)} = 12 \]
- The same calculation applies for an atom in Layer B, confirming that the coordination number remains consistent.

5. **Conclusion**:
- Thus, the coordination number in an ABAB type arrangement (Hexagonal Close Packing) is **12**.