What is Co-ordination number

Options

A. 3
B. 4
C. 6
D. 8

Answer

### Step-by-Step Solution:

1. **Definition of Coordination Number**:
- The coordination number of an atom in a molecule or a polyatomic ion refers to the number of other atoms (or ligands) that are directly bonded to it.

2. **Understanding Ligands**:
- Ligands are atoms, ions, or molecules that can donate a pair of electrons to a central atom (usually a metal) to form a coordination complex.

3. **Example of Coordination Number**:
- Consider the complex ion \( \text{Cr(NH}_3\text{)}_2\text{Cl}_2\text{Br}^{-} \).
- In this complex, the central atom is chromium (Cr).

4. **Determining the Oxidation State**:
- The oxidation state of chromium in this complex can be determined to be +3.

5. **Counting the Ligands**:
- In the given complex, there are 2 ammonia (NH₃) ligands, 2 chloride (Cl) ligands, and 1 bromide (Br) ligand.
- Thus, the total number of ligands is calculated as follows:
- 2 (from NH₃) + 2 (from Cl) + 2 (from Br) = 6 ligands.

6. **Conclusion**:
- Therefore, the coordination number of the chromium in this complex is 6, indicating that it is hexa-coordinated.