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

Options

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

Explanation

**Step-by-Step Solution:**

1. **Understanding the Properties of Mercury:**
- Mercury is unique among metals as it is liquid at room temperature (0°C). This property can be attributed to its atomic structure and bonding characteristics.

2. **High Ionization Energy:**
- Mercury has a high ionization energy, which means that it requires a significant amount of energy to remove an electron from its outer shell. This high ionization energy prevents mercury atoms from easily losing electrons and forming strong metallic bonds.

3. **Weak Interatomic Metallic Bonding:**
- Due to the high ionization energy, mercury atoms do not share electrons with each other effectively. This results in weak interatomic metallic bonding. In metals, strong metallic bonding typically leads to solid states, but in mercury, the weak bonding allows it to remain liquid.

4. **Fully Filled d Orbitals:**
- Mercury has fully filled d orbitals. The presence of fully filled d orbitals means that d-d overlap (which can enhance metallic bonding) does not occur. This lack of effective overlap contributes further to the weak metallic bonding in mercury.

5. **Conclusion:**
- The combination of high ionization energy and weak interatomic metallic bonding, along with the fully filled d orbitals, leads to mercury being liquid at 0°C. Therefore, the correct answer to the question is that mercury's liquid state at this temperature is due to its high ionization energy and weak metallic bonding.

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

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