**Number of Particles per Unit Cell in Different Crystal Structures**

**Simple Cubic Structure:**

In a simple cubic structure, each corner of the cube contains one particle, and there are no particles in the center or on the faces of the cube. Therefore, the total number of particles per unit cell in a simple cubic structure is 1.

**BCC Structure:**

In a body-centered cubic (BCC) structure, each corner of the cube contains one particle, similar to the simple cubic structure. In addition, there is one particle located at the center of the cube. Hence, the total number of particles per unit cell in a BCC structure is 2.

**FCC Structure:**

In a face-centered cubic (FCC) structure, each corner of the cube contains one particle, same as in the simple cubic and BCC structures. Additionally, there is a particle located at the center of each face of the cube. Therefore, the total number of particles per unit cell in a FCC structure is 4.

**Hexagonal Structure:**

In a hexagonal structure, the number of particles per unit cell depends on the specific arrangement of atoms. However, in most cases, hexagonal structures have more than one particle per unit cell. The exact number of particles per unit cell in a hexagonal structure can vary depending on the specific crystal lattice arrangement and the unit cell chosen.

**Efficiency of Packing:**

The efficiency of packing refers to the amount of space occupied by particles in a crystal structure. It can be calculated by dividing the volume occupied by the particles by the total volume of the unit cell.

Based on the number of particles per unit cell, we can compare the efficiency of packing as follows:

- Simple Cubic: With only one particle per unit cell, the packing efficiency is relatively low.
- BCC: With two particles per unit cell, the packing efficiency is higher than in the simple cubic structure but still not the most efficient.
- FCC: With four particles per unit cell, the packing efficiency is higher than in both simple cubic and BCC structures. The FCC structure provides a more efficient packing arrangement.
- Hexagonal: The number of particles per unit cell in a hexagonal structure can vary, so it is difficult to make a general statement about its efficiency of packing.

In summary, the FCC structure has the highest number of particles per unit cell, indicating a more efficient packing arrangement compared to the simple cubic and BCC structures. The hexagonal structure's efficiency of packing can vary depending on the specific arrangement, so it cannot be conclusively compared to the other structures in terms of particle number alone.