Question

Coordination number in BCC crystals is

• A. 6
• B. 4
• C. 8
• D. 12

Hint:

Remember: BCC = 8, FCC = 12, Simple Cubic = 6 (coordination numbers).

Answer 1

The Correct Option is C

Step 1: Understanding coordination number.
The coordination number is the number of nearest neighboring atoms surrounding a given atom in a crystal structure.
Step 2: Structure of BCC crystal.
In a Body-Centered Cubic (BCC) structure, each atom at the center of the cube is surrounded by atoms located at the eight corners of the cube.
Step 3: Counting nearest neighbors.
Each atom in a BCC lattice has eight nearest neighboring atoms at equal distances.
Step 4: Conclusion.
Therefore, the coordination number of a BCC crystal is 8.

Answer 2

Coordination number is easiest to pin down by comparing it across the standard crystal structures, since each has a well-known, distinct value.

1. 6: A coordination number of 6 belongs to the simple cubic (SC) structure, where each atom sits at a cube corner with one nearest neighbour along each of the six face directions. BCC has a different, more efficiently packed arrangement, so 6 does not apply to it.
2. 4: A coordination number of 4 corresponds to structures like diamond cubic, where each atom is tetrahedrally bonded to four neighbours. This is a much more open structure than BCC and does not match its packing.
3. 8: In BCC, the atom sitting at the body center of the cube is equidistant from all eight corner atoms of that same cube, and by symmetry every corner atom likewise has eight equidistant neighbours (from the body centers of the eight cubes surrounding it). This gives BCC its characteristic coordination number of 8.
4. 12: A coordination number of 12 belongs to the close-packed structures, FCC and HCP, where atoms are packed as efficiently as geometrically possible. BCC is less densely packed than these structures, so its coordination number is lower than 12.

Matching BCC's actual atomic arrangement to the known coordination numbers of standard structures confirms the value is 8.

Therefore, the correct answer is 8.