The co-ordination number of a crystal structure refers to the number of nearest neighboring atoms surrounding a central atom. In the case of a body-centered cubic (BCC) crystal structure, the co-ordination number is 8.

Explanation:

1. Body-Centered Cubic (BCC) Structure:
- The BCC crystal structure is one of the common arrangements of atoms in a solid material.
- In this structure, the atoms are arranged in a cubic lattice, with an additional atom located in the center of the cube.
- Each corner of the cube is shared by 8 adjacent unit cells, while the center atom is not shared with any other unit cells.

2. Identifying Nearest Neighbors:
- To determine the co-ordination number of a BCC structure, we need to identify the nearest neighboring atoms surrounding a central atom.
- In this case, the central atom is located at the center of the cube.
- The nearest neighboring atoms are the 8 atoms situated at the corners of the cube.
- Each corner atom is shared by 8 unit cells, with one unit cell belonging to the central atom.

3. Co-ordination Number:
- The co-ordination number is the total number of nearest neighboring atoms surrounding a central atom.
- In the BCC crystal structure, there are 8 corner atoms surrounding the central atom.
- Therefore, the co-ordination number of a BCC crystal structure is 8.

In summary, the co-ordination number of a BCC crystal structure is 8 because there are 8 corner atoms surrounding the central atom.