Every atom or ion that forms an fcc unit cell is surrounded by

Options

A. Six `OVs` and eight `TVs`.
B. Eight `OVs` and Six `TVs`.
C. Six `OVs` and six `TVs`.
D. Eight `OVs` and four `TVs`.

Explanation

To solve the question "Every atom or ion that forms an FCC unit cell is surrounded by," we need to analyze the structure of the Face-Centered Cubic (FCC) unit cell and understand how the atoms and voids are arranged.

### Step-by-Step Solution:

1. **Understanding the FCC Structure**:
- In an FCC unit cell, atoms are located at each of the eight corners of the cube and at the centers of each of the six faces.

2. **Counting the Atoms**:
- Each corner atom is shared among eight adjacent unit cells, contributing \( \frac{1}{8} \) of an atom per unit cell.
- Each face-centered atom is shared between two unit cells, contributing \( \frac{1}{2} \) of an atom per unit cell.
- Therefore, the total number of atoms in an FCC unit cell is:
\[ \text{Total atoms} = 8 \times \frac{1}{8} + 6 \times \frac{1}{2} = 1 + 3 = 4 \text{ atoms} \]

3. **Identifying Tetrahedral Voids**:
- In an FCC unit cell, each atom is surrounded by tetrahedral voids. Each atom has 8 tetrahedral voids around it.
- The tetrahedral voids are located at a distance of \( \frac{\sqrt{3}}{4} a \) from the corner atom, where \( a \) is the edge length of the cube.

4. **Identifying Octahedral Voids**:
- Each atom is also surrounded by octahedral voids. There is one octahedral void located at the center of the unit cell and one at the center of each edge.
- Each corner atom is surrounded by 6 octahedral voids.

5. **Conclusion**:
- Therefore, every atom or ion in an FCC unit cell is surrounded by **6 octahedral voids** and **8 tetrahedral voids**.

### Final Answer:
Every atom or ion that forms an FCC unit cell is surrounded by **6 octahedral voids and 8 tetrahedral voids**.

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