Mercury is the only metal which is liquid at `0^(@)C`.this is due to

Options

A. Very high ionisation energy and weak metallic bond
B. Low ionisation potential
C. High atomic weight
D. High vapour pressure

Answer

To solve the question regarding why mercury is the only metal that is liquid at 0°C, we can break down the reasoning step by step.

### Step-by-Step Solution:

1. **Understanding Mercury's Properties**:
- Mercury (Hg) is unique among metals as it remains liquid at room temperature and even at 0°C.

2. **Electronic Configuration**:
- The electronic configuration of mercury is given as:
\[ \text{Hg: [Xe] 4f}^{14} 5d^{10} 6s^{2} \]
- This configuration indicates that mercury has a filled d-subshell (5d) and a filled f-subshell (4f).

3. **High Ionization Energy**:
- Mercury has a very high ionization energy. This high ionization energy is due to the stabilization of the 6s electrons.
- The 6s electrons are held tightly due to the effective nuclear charge, which is increased by the poor shielding effect of the 4f electrons.

4. **Poor Shielding Effect**:
- The 4f electrons do not shield the nucleus effectively, leading to a higher effective nuclear charge felt by the outer electrons.
- This results in higher ionization energy, making it difficult for mercury atoms to lose electrons and form metallic bonds.

5. **Weak Metallic Bonding**:
- Because of the high ionization energy, mercury atoms do not easily share electrons with each other, resulting in weak interatomic metallic bonds.
- Additionally, since mercury has fully filled d-orbitals, d-d overlap cannot occur, further contributing to the weak metallic bonding.

6. **Conclusion**:
- The combination of high ionization energy and weak metallic bonds results in mercury being a liquid at 0°C, as the weak metallic bonds are not strong enough to hold the atoms in a solid state.

### Final Answer:
Mercury is the only metal that is liquid at 0°C due to its very high ionization energy and weak metallic bonds.