“There is a place. Like no place on Earth. A land full of wonder, mystery, and danger! Some say to survive it: You need to be as mad as a hatter. Which luckily I am.” Alice in Wonderland, Lewis Carroll
The phrase, “Mad as a hatter”, to describe a person who is not mentally sound originated in the 1700s. At the time, mercury was used in the production of felt, a material which was often used in hat manufacturing. The people who worked at hat factories would sometimes develop dementia through gradual accumulation of trace amounts of mercury in their bodies – this process is called mercury poisoning. Despite its dangerous effects on the body, the physical and chemical properties of mercury have resulted in its continued use in a range of applications. Because of its high density, mercury is used in barometers – an instrument used for measuring atmospheric pressure. It was used in thermometers because of its high rate of thermal expansion which is fairly constant over a large temperature range. Perhaps one of mercury’s most interesting physical property is its exceptionally low melting point compared to other metals, the explanation to which lies in its electronic configuration.
For mercury, the 1s, 2s, 2p, 3s, 3p, 3d, 4s, 4p, 4d, 4f, 5s, 5p, 5d, and 6s subshells are filled with electrons. The filling of the 6s subshell is important, its high stability results in the formation of weak bonds between mercury atoms. Gold on the other hand, which is one place to the left of mercury in the periodic table, has only one electron in the 6s subshell making it far less stable. The electrons in this 6s subshell are more easily removed and shared between gold atoms resulting in strong metallic bonding.
But many elements in the periodic table have full outer subshells, what makes the 6s subshell of mercury so stable? The answer to this question is twofold. The lanthanide contraction is a term used to describe the larger than expected decrease in ionic radii across the lanthanide series which results in the smaller than expected ionic radii of subsequent elements in the periodic table. The effect can be explained by the poor electronic shielding of the 4f electrons causing the 6s to be drawn more closely to the nucleus and hence become more stable. However, the lanthanide effect does not fully account for the anomalously low melting point of mercury. The complete answer lies with the relativistic effect.
Relativistic effects are important for the heavier elements of the periodic table where the electrons are travelling near to the speed of light. This leads to a relativistic increase in the mass of the electron according to:
Mrel = Me/sqrt(1-(Ve/c)^2)
where Me, Ve and c are the rest mass of the electron, its velocity, and speed of light respectively.
This increase in mass leads to a contraction of atomic orbitals according to:
Arel/Ao = sqrt(1-(Ve/c)^2)
where Ao is the radius of an atom at rest.
This additional effect results in the high stability of mercury atoms and leads on to explain some of its strange physical and chemical properties.