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 can follow these steps:

### Step-by-Step Solution:

1. **Understanding ABAB Arrangement**:
- The ABAB arrangement refers to a type of close packing where layers alternate between two different types of atoms or ions, denoted as A and B.

2. **Identifying the Structure**:
- This arrangement is characteristic of Hexagonal Close Packing (HCP). In HCP, the layers are stacked in a specific manner where each layer A is followed by a layer B and then another layer A, and so on.

3. **Visualizing the Layers**:
- Imagine a 2D representation of the layers. The first layer (A) forms a hexagonal shape. The second layer (B) fits into the depressions of the first layer (A).

4. **Analyzing the Coordination of an Atom**:
- Let’s consider an atom (C) located in one of the A layers. This atom (C) will be surrounded by other atoms from the same layer (A) and the adjacent layers (B).

5. **Counting Neighbors in Layer A**:
- In the same layer (A), atom C is surrounded by 6 other atoms. This is because in a hexagonal arrangement, each atom is surrounded by 6 others.

6. **Counting Neighbors in Layer B**:
- Atom C is also surrounded by atoms from the layer B above it and the layer B below it. Each of these layers contributes 3 atoms to the coordination of atom C (3 from the layer above and 3 from the layer below).

7. **Calculating Total Coordination Number**:
- Therefore, the total coordination number for atom C is:
\[ \text{Coordination Number} = \text{Atoms from Layer A} + \text{Atoms from Layer B (above)} + \text{Atoms from Layer B (below)} \]
\[ = 6 + 3 + 3 = 12 \]

8. **Conclusion**:
- The coordination number in an ABAB type arrangement (HCP) is 12.

### Final Answer:
The coordination number in ABAB type arrangement is **12**.

---