Why is mercury liquid?

Mercury is liquid at room temperature due to its unique electronic configuration status and special contraction in its atomic size. It is the silver-white, shiny metal liquid and one of the only two elements in the periodic table that exist as liquids at room temperature.

The main factors for the existence of mercury as a liquid are:

1. Atomic size contraction.
2. Weak intramolecular bonds among mercuric atoms.
3. Electronic configuration.

Contraction of atomic size is a phenomenon usually seen in periods in the periodic table. Mercury is present in the group (2-B) of the periodic table which corresponds to its smaller size due to ‘along a period contraction’ in atomic sizes.

Besides the period contraction in the size of atoms, mercury, and other period members experience another size contraction known as ‘the lanthanide contraction’ phenomenon. It is the exceptional decrease in ionic radii of elements due to the poor shielding effect of 4f electrons.

These contractions make the Hg-Hg bond too weak to withstand a solid configuration, making it an only naturally occurring liquid metal.

The electronic configuration of mercury,

Hg⁸⁰ = [Xe]⁵⁴, 4f¹⁴, 5d¹⁰, 6s²

This matches the electronic configurations of noble gases, so it is not easy for mercury to lose electrons and gain stability by forming a variety of compounds.

In short, electrons of mercury (Hg) are too near the nucleus, and too fast or too unpredictable to move that they cannot be contained in a strong bond, i.e. solids. This concept of weak bonding makes a good explanation of mercury, being a liquid at room temperature.  

Uses of Mercury (Hg) as liquid

1. Mercury is used in thermometers. It is a place where freezing and boiling need to be strictly avoided or thermometer liquids may be stuck, leaving thermometers useless.
2. Being a liquid, mercury dissolves gold (Au), zinc (Zn), and many other metals to make amalgams. Nowadays, they are being used in dental fillings and have many other applications.
3. From vapor forms of mercury, electricity is passed in phosphor tubes to produce light.