Sodium metal is soft but Iron is a hard metal. This difference arises due to.

Options

A. Sodium has lesser atomic mass than iron
B. Sodium has only one valence electron where as iron has more
C. There is one unpaired electron is sodium but four unpired electrons in iron
D. Sodium reacts readily with air and moisture but iron is resistanct to action of water

Answer

To understand why sodium metal is soft while iron is hard, we can analyze the properties of both metals based on their electronic configurations and the nature of metallic bonding.

### Step-by-Step Solution:

1. **Identify the Group and Electronic Configuration of Sodium:**
- Sodium (Na) belongs to Group 1 of the periodic table.
- Its atomic number is 11, which gives it the electronic configuration of 1s² 2s² 2p⁶ 3s¹.
- This means sodium has one valence electron in its outermost shell (3s orbital).

2. **Understanding Sodium's Metallic Bonding:**
- The presence of only one valence electron means that sodium can easily lose this electron.
- This single electron participates in metallic bonding, which is relatively weak due to the limited number of electrons involved.
- Additionally, sodium has a larger atomic size, which contributes to weaker metallic bonds.

3. **Identify the Group and Electronic Configuration of Iron:**
- Iron (Fe) belongs to Group 8 and has an atomic number of 26.
- Its electronic configuration is [Ar] 3d⁶ 4s².
- In this configuration, iron has a total of 4 unpaired electrons (2 in the 4s orbital and 6 in the 3d orbital).

4. **Understanding Iron's Metallic Bonding:**
- The presence of four unpaired electrons allows iron to form strong metallic bonds.
- These strong bonds arise because more electrons are available to participate in bonding, leading to a more stable and rigid structure.

5. **Conclusion:**
- The softness of sodium is attributed to its weak metallic bonding due to having only one unpaired electron and its larger atomic size.
- In contrast, the hardness of iron is due to strong metallic bonding facilitated by its four unpaired electrons.

### Final Answer:
The difference in hardness between sodium and iron arises due to the number of unpaired electrons available for metallic bonding. Sodium has one unpaired electron, leading to weak metallic bonds, while iron has four unpaired electrons, resulting in strong metallic bonds.