Short Answer

Gold is yellow and mercury is liquid at room temperature due to relativistic effects in heavy elements.

• In gold, relativistic contraction of 6s electrons changes the energy levels of d→s transitions, causing absorption of blue light, which gives the metal its yellow color.
• In mercury, the same contraction weakens metallic bonding, reducing cohesion between atoms, so mercury remains liquid at room temperature.

Detailed Explanation :

Relativistic Effects in Heavy Elements

Relativistic effects occur when inner electrons of heavy atoms move at speeds close to the speed of light.

• This effect increases the mass of electrons, contracts s-orbitals, and slightly expands d- and f-orbitals.
• It significantly influences physical and chemical properties of heavy elements like gold (Au, Z=79) and mercury (Hg, Z=80).

Gold’s Yellow Color

1. Electronic Configuration:
   • Gold: [Xe] 4f¹⁴ 5d¹⁰ 6s¹
2. Effect on d→s Transition:
   • Normally, s and d orbitals have larger energy difference, which absorbs ultraviolet light.
   • Relativistic contraction lowers the 6s orbital energy and raises 5d orbital energy.
   • The energy difference now corresponds to visible light absorption in the blue region (~2.4 eV).
3. Perceived Color:
   • Absorption of blue light → remaining reflected light is yellow, giving gold its characteristic color.
4. Contrast with Silver:
   • Silver does not have strong relativistic effects → d→s transition absorbs ultraviolet → appears silvery white.

Mercury’s Liquid State

1. Electronic Configuration:
   • Mercury: [Xe] 4f¹⁴ 5d¹⁰ 6s²
2. Weak Metallic Bonding:
   • Relativistic contraction of 6s electrons → electrons held closer to nucleus, not delocalized for bonding.
   • Weaker overlap of orbitals → low cohesion between atoms.
3. Physical Consequence:
   • Mercury atoms cannot form a strong metallic lattice → low melting point (~ -39°C)
   • Remains liquid at room temperature, unlike other metals.
4. Comparison with Other Group 12 Metals:
   • Cadmium (Cd) and Zinc (Zn) → solid at room temperature because relativistic effects are weaker.

Other Factors Contributing

1. Spin-Orbit Coupling:
   • Heavy elements experience stronger spin-orbit interactions, slightly altering orbital energies.
2. d-Electron Shielding:
   • Poor shielding by d-electrons enhances relativistic effects on 6s orbital contraction.
3. Thermodynamic Effects:
   • Reduced metallic bond strength in mercury → low lattice energy, reinforcing liquid state.

Significance

• Explains physical anomalies in heavy elements.
• Relativistic effects impact color, melting points, chemical reactivity, and other material properties.
• Critical in nanotechnology, metallurgy, and jewelry (gold) and industrial applications (mercury).

Conclusion

Gold is yellow because relativistic contraction of 6s electrons alters the d→s transition energy, reflecting yellow light. Mercury is liquid at room temperature because the same contraction weakens metallic bonding, preventing formation of a solid lattice. These anomalies demonstrate how relativistic effects influence electronic structure and macroscopic properties in heavy elements, highlighting the importance of these effects in chemistry and material science.