Why is mercury liquid?
Dear Mr Thurnherr,
My reply is based on a somewhat older article from the Journal for Chemical Education from 1991. I shall keep my answer general at first, then become more specific, as I can see that you have a background in physics.
General answer
Mercury is liquid at room temperature because the individual mercury atoms do not bind strongly to one another and therefore do not form strong metallic bonds. This means that mercury must be cooled to a lower temperature to become a solid metal – to be precise: below its melting point of –39 °C.
Whether a substance is solid, liquid or gaseous depends on the temperature. The higher the temperature, the faster the atoms or molecules move and the less ordered the material is. How high the melting point (solid to liquid) and the boiling point (liquid to gaseous) are depends on how strongly the individual atoms or molecules attract one another. If the attraction is strong, the melting and boiling points are high. If the attraction is weak, the melting and boiling points are low.
All metals have metallic bonds between their atoms, through which the atoms share electrons with one another. Mercury does not readily share its electrons. Consequently, the metallic bond is weak and the melting point is so low. Mercury behaves almost a little like the noble gases neon, argon, xenon, and so on.
Specific answer
The reason for mercury’s weak metallic bond lies in its electron configuration. This becomes clear when comparing gold (Au) and mercury (Hg): The two metals are neighbours, but behave quite differently.
Electron configuration:
- Mercury (Hg): [Xe] 4f14 5d10 6s2
- Gold (Au): [Xe] 4f14 5d10 6s1
Mercury has a fully filled 6s shell (i.e. one more electron than gold). In summary, this has three consequences:
- Lanthanide contraction: Electrons in f-orbitals shield the positive charge of the nucleus less effectively than electrons in other orbitals. This means that the remaining electrons feel a stronger attraction to the nucleus, causing the atom to shrink. A smaller atom with more tightly bound electrons is less willing to share these electrons, so the attraction between mercury atoms is reduced.
- Relativistic effects: According to Einstein’s theory of relativity, the mass of a moving object increases with its speed. The larger an atom, the further away from the nucleus and the faster the electrons in the outer shells are. This increases the effective mass of these electrons, which in turn leads to a reduction in the size of the atom. As a result, mercury is much smaller and binds its electrons more tightly, which reduces the attraction between one mercury atom and another.
- Fully filled 6s orbital: Because the 6s orbital is completely filled (the electrons are paired), there is little reason for the mercury atom to share these electrons. This is another reason why the attraction between mercury atoms is reduced.
As a result, mercury is held together mainly by van der Waals forces, which are weak. This is why mercury’s melting point is so low and why it is liquid at room temperature.
With kind regards and in the hope that I have provided a more detailed answer,
Michael Lerch
www.lerchlab.com
Reference:
- Norrby, Lars J. 1991. “Why is mercury liquid? Or, why do relativistic effects not get into chemistry textbooks?”. Journal of Chemical Education 68(2): 110. Online.