Why Is Mercury Liquid at Room Temperature?

An international research team that includes scientists from Heidelberg University have solved the "mystery" of mercury with the help of computer experiments. Using simulations and numerical methods, the team sought to answer the question of why this metal always occurs in liquid form under normal environmental conditions. The researchers from New Zealand, France, and Heidelberg were able to prove that the low melting point is to be found in mercury's special electron structure, something that can only be explained through Albert Einstein's special theory of relativity (STR). The results of their research were published in the journal “Angewandte Chemie".

“The properties of mercury have long puzzled theoretical chemists. Its aggregate state under normal conditions is always liquid, unlike other metals such as zinc, gold or copper, which need a lot of heat to melt”, explains physicist Dr. Michael Wormit, a researcher in theoretical chemistry at the Interdisciplinary Center for Scientific Computing (IWR) of Heidelberg University. “Mercury frequently behaves more like an inert gas than a metal.”

Researchers have long suspected that mercury's peculiarities are rooted in the effects of the special theory of relativity, but they have not been able to prove it quantitatively until now. Einstein's theory describes the properties of very high-velocity matter, which exists in the mercury atom in the form of 82 electrons. The electron structure of the mercury atom therefore differs from that of lighter atoms, where these effects play a lesser role. Together with Dr. Florent Calvo (University of Lyon, France), Dr. Elke Pahl and Prof. Dr. Peter Schwerdtfeger (both of Massey University, Auckland, New Zealand), Dr. Wormit built a computer model of the atomic structure of mercury with its nucleus and electrons. Using computer simulations, the team studied the interaction of mercury atoms at various pressures and temperatures.

“For a long time computers simply weren't powerful enough for these types of simulations and calculations”, states the Heidelberg scientist, who also used the Monte Carlo Simulation in his work. This stochastic mathematical process is based on random experiments run many times over. Probability theory is used to produce numerical results to answer questions where a deterministic calculation is impossible.

“Sufficient computing capacity made our approach realisable for the first time, and we were able to demonstrate that the relativistic effects are critically important for the simulation of mercury materials.  Without these effects the melting point of crystalline or solid mercury is about 105 degrees Celsius higher, and it wouldn't be liquid at room temperature, but solid”, explains Wormit.

Original publication:
F. Calvo, E. Pahl, M. Wormit, P. Schwerdtfeger: Erklärung des niedrigen Schmelzpunkts von Quecksilber mit relativistischen Effekten, Angew. Chem. 2013, 125, 7731-7734, doi: 10.1002/ange.201302742 (German version)

F. Calvo, E. Pahl, M. Wormit, P. Schwerdtfeger: Evidence for Low-Temperature Melting of Mercury owing to Relativity, Angew. Chem. Int. Ed. 2013, 52, 7583-7585, doi: 10.1002/anie.201302742 (English version)