The radius of chromium atom is `1.25Å`. If it crystallises in body centred cubic lattice, calculate the length of the edge of the unit cell.

Explanation

To solve the problem of finding the length of the edge of the unit cell for chromium crystallizing in a body-centered cubic (BCC) lattice, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the given data**:
- The radius of the chromium atom (R) = 1.25 Å.

2. **Understand the structure of BCC**:
- In a body-centered cubic lattice, there are atoms at each corner of the cube and one atom at the center of the cube.

3. **Establish the relationship between the radius (R) and the edge length (A)**:
- For a BCC structure, the relationship between the radius of the atom (R) and the edge length of the unit cell (A) is given by the formula:
\[ A = \frac{4R}{\sqrt{3}} \]

4. **Substitute the value of R into the formula**:
- Plugging in the value of R:
\[ A = \frac{4 \times 1.25}{\sqrt{3}} \]

5. **Calculate the numerical value**:
- First, calculate \(4 \times 1.25\):
\[ 4 \times 1.25 = 5.0 \]
- Now, calculate \(\sqrt{3}\) (approximately 1.732):
\[ A = \frac{5.0}{1.732} \]
- Performing the division:
\[ A \approx 2.89 \text{ Å} \]

6. **Round off the answer**:
- The length of the edge of the unit cell (A) can be approximated to:
\[ A \approx 2.9 \text{ Å} \]

### Final Answer:
The length of the edge of the unit cell for chromium in a body-centered cubic lattice is approximately **2.9 Å**.