The coordination number of h.c.p. or c.c.p. structure is 12, that is, one atom touches 12 other atoms. The 12 atoms lie as

Options

A. 8 atoms are on the same plane, 2 above and 2 below the plane
B. 6 atoms are on the same plane, 3 above and 3 below the plane
C. 4 atoms are on the same plane, 4 above and 4 below the plane
D. 2 atoms are on the same plane, 5 above and 5 below the plane

Answer

To determine how the 12 atoms surround a central atom in a hexagonal close-packed (h.c.p.) or cubic close-packed (c.c.p.) structure, we can break down the arrangement step by step.

### Step-by-Step Solution:

1. **Understanding Coordination Number**:
- The coordination number of a structure indicates how many atoms are directly touching a central atom. In h.c.p. and c.c.p. structures, the coordination number is 12.

2. **Visualizing the Arrangement**:
- Imagine a central atom (let's call it the green atom). This atom is surrounded by other atoms.

3. **Atoms in the Same Plane**:
- First, consider the atoms that are in the same horizontal plane as the central atom. There are 6 atoms that surround the central atom in this plane. These can be visualized as forming a hexagon around the central atom.

4. **Atoms Above and Below**:
- Next, we need to account for the atoms that are positioned above and below the central atom.
- There are 3 atoms directly above the central atom and 3 atoms directly below it.

5. **Counting the Atoms**:
- Now, let's count the total number of atoms that are in contact with the central atom:
- 6 atoms in the same plane
- 3 atoms above
- 3 atoms below
- Adding these together gives us:
\[ 6 + 3 + 3 = 12 \]

6. **Conclusion**:
- Therefore, the arrangement of the 12 atoms around the central atom is such that there are 6 atoms in the same plane, 3 atoms above, and 3 atoms below the central atom.

### Final Answer:
The 12 atoms lie as 6 atoms in the same plane, 3 above, and 3 below the central atom.