The factor which makes a solid to have a low density is

Options

A. close packing
B. large atomic radius
C. high atomic mass
D. high valency

Answer

To determine the factor that makes a solid have a low density, we need to analyze the options provided in the question. Density is defined as mass per unit volume (Density = Mass/Volume).

### Step-by-Step Solution:

1. **Understanding Density**:
- Density is calculated using the formula:
\[ \text{Density} = \frac{\text{Mass}}{\text{Volume}} \]
- A solid with a low density will have either a low mass or a high volume.

2. **Analyzing Option A: Close Packing**:
- Close packing refers to the arrangement of particles in a solid.
- In close-packed structures, particles are closely packed together, which means there are fewer voids (empty spaces).
- This results in a higher mass for a given volume, leading to a higher density.
- Therefore, close packing is not responsible for low density. **(Incorrect)**

3. **Analyzing Option B: Large Atomic Radius**:
- A large atomic radius means that the atoms occupy a larger volume.
- If the volume increases while the mass remains relatively constant, the density decreases.
- Thus, a large atomic radius contributes to low density. **(Correct)**

4. **Analyzing Option C: High Atomic Mass**:
- High atomic mass means that the mass of the atoms is greater.
- Since density is directly proportional to mass, an increase in atomic mass would lead to an increase in density.
- Therefore, high atomic mass does not contribute to low density. **(Incorrect)**

5. **Analyzing Option D: High Valency**:
- High valency indicates stronger bonding and attraction between atoms.
- This often leads to closer packing of atoms, which increases density.
- Hence, high valency does not contribute to low density. **(Incorrect)**

### Conclusion:
The factor that makes a solid have a low density is **B: Large Atomic Radius**.