What is meant by co-ordintion number (C.N.) ?

Explanation

### Step-by-Step Solution:

1. **Definition of Coordination Number**:
- The coordination number (C.N.) refers to the total number of nearest neighbor atoms surrounding a central atom in a solid. It is a measure of how many atoms are directly adjacent to a given atom in a crystal lattice.

2. **Context in Solid State**:
- In the context of solid-state chemistry, the coordination number is significant as it helps to describe the structure and stability of the solid. Different arrangements of atoms lead to different coordination numbers.

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