The alkali metals are soft due to-

Options

A. B.C.C. structure
B. 68% packing efficiency
C. Weak metallic bond
D. All the above

Answer

**Step-by-Step Solution:**

1. **Identify the Group of Alkali Metals**:
- Alkali metals belong to Group 1 of the periodic table and are classified as S-block elements.

2. **Understand the Structure of Alkali Metals**:
- Alkali metals have a body-centered cubic (BCC) structure. This arrangement affects their physical properties.

3. **Examine the Nature of Metallic Bonds**:
- The metallic bonds in alkali metals are relatively weak compared to other metals. This is due to the presence of a single valence electron, which does not hold the metal atoms together as strongly.

4. **Consider Coordination Number**:
- Alkali metals have a low coordination number (CN), which contributes to their softness. A low coordination number means that there are fewer neighboring atoms surrounding each atom, leading to weaker interactions.

5. **Evaluate Packing Efficiency**:
- The packing efficiency (η) of alkali metals is about 68%. This means that there is a significant amount of empty space in the structure, which also contributes to their softness.

6. **Conclusion**:
- The softness of alkali metals can be attributed to their BCC structure, weak metallic bonds, low coordination number, and packing efficiency. Therefore, the correct answer is that alkali metals are soft due to all of the above factors.

**Final Answer**: The alkali metals are soft due to their body-centered cubic structure, weak metallic bonds, low coordination number, and packing efficiency. Thus, the correct option is D: All of the above.

---