Why Mercury Is Liquid: The Scientific Explanation Behind It

Mercury is a metal, but it is liquid at room temperature. With a melting point around minus 38.8 degrees Celsius, the element is one of only two elements that are liquid at room temperature. The other is bromine, but bromine is not a metal. Then arises an interesting question: why is mercury different from other metals? The answer relates to mercury’s position in the periodic table, as well as how its atoms bond with one another. Generally, the melting point of a substance depends on the strength of the interatomic bonds. Chemist and senior chemistry lecturer at the UK Ministry of Defence, Zoe Ashbridge, explains that the stronger the interatomic bonds, the more energy is required to break them. In metals, atoms are bound by metallic bonds. In this structure, metal atoms form a lattice consisting of positively charged ions, while free electrons move around them like a ‘sea of electrons’. This structure gives metals several characteristic properties, but it is this electrostatic attraction that determines the melting point. In theory, mercury is in Group 12 of the periodic table. That means it has 12 valence electrons that could be used to form metallic bonds. However, there is a problem: all those electrons reside in a subshell that is already full. According to Ashbridge: ‘When the subshell is full, electrons become more stable and more difficult to delocalise. This makes mercury reluctant to share electrons, even with other mercury atoms.’ Consequently, the metallic bonds in mercury are much weaker than in other metals. But this explanation is not enough. If you follow trends in the periodic table, mercury should still have a melting point of around 130°C, which would make it solid at room temperature. The reality, however, is not so.