Mercury is the only liquid metal at room temperature due to:

Very high ionisation enthalpy and weak metallic bond

The correct answer is Option (1) → Very high ionisation enthalpy and weak metallic bond

Let us delve into the properties of mercury that contribute to its unique status as the only liquid metal at room temperature.

Physical Properties of Mercury

Liquid State at Room Temperature: Mercury (Hg) remains in a liquid state at temperatures ranging from -38.83 °C to 356.73 °C. This characteristic sets it apart from other metals, which are typically solid at room temperature.

Atomic Structure and Electron Configuration

Electron Configuration: Mercury has the atomic number 80, with the electron configuration \([Xe] 4f^{14} 5d^{10} 6s^2\). The filled d and f orbitals contribute to its chemical behavior.

High Ionization Energy: The first ionization energy of mercury is relatively high (10.4 eV). This means it requires a significant amount of energy to remove an electron, leading to weak metallic bonding.

Metallic Bonding in Mercury

Weak Metallic Bonds: In metals, atoms are held together by metallic bonds, which involve a "sea of delocalized electrons" that allow for conductivity and malleability. In mercury, the metallic bonds are weaker due to:

High Ionization Energy: The high energy required to ionize mercury means that the bonding electrons are less available for forming strong metallic bonds.

Heavy Atomic Mass: The larger atomic size and mass of mercury contribute to the spatial arrangement of atoms, leading to weaker interactions compared to lighter metals.

Comparison with Other Metals

Stronger Metallic Bonds in Solids: Most metals have lower ionization energies and thus form stronger metallic bonds. For instance, sodium and iron have lower ionization energies and are solid at room temperature due to the robust bonding.

Effects of Temperature: As temperature increases, the kinetic energy of atoms rises, allowing solids to transition to liquids. However, for metals with strong metallic bonds, this transition occurs at much higher temperatures than for mercury.

Role of Temperature and Vapor Pressure

High Vapor Pressure: Mercury has a higher vapor pressure than many metals at room temperature, which also contributes to its unique properties. The vapor pressure of mercury at room temperature allows it to exist as a liquid without excessive evaporation.

Thermal Expansion: Mercury expands uniformly when heated, making it useful in thermometers and other applications.

Summary of Key Points

Ionization Energy and Bonding: The high ionization energy and weak metallic bonding in mercury allow it to remain liquid at room temperature.

Unique Electron Configuration: The filled d and f orbitals influence mercury’s bonding characteristics, leading to its liquid state.

Comparative Analysis: Compared to other metals, mercury's weak bonding and higher vapor pressure explain why it is the only metal that exists in a liquid state at room temperature.

Conclusion

Mercury's liquid state at room temperature can be attributed to a combination of its high ionization energy and weak metallic bonds, alongside its unique atomic structure. This allows mercury to maintain its liquid form in conditions where most other metals are solid. Understanding these properties provides insight into the unusual behavior of mercury and its applications in various fields.