Mercury is a liquid at room temperature, while other metals are solids. Have you ever wondered what makes mercury special? The quick answer is that mercury is a liquid because its atoms do not readily share electrons with other mercury atoms. Here’s a closer look at how it works.
- Mercury is a liquid because it doesn’t share its electrons with other mercury atoms very well. Basically, it acts like the metallic equivalent to a noble gas.
- The large number of protons in the atomic nucleus attracts the electrons in what is called lanthanide contraction. Relativistic effects play a role.
- The filled 4f-subshell only poorly shields the 6s shell, drawing the valence electrons closer to the nucleus than in other metals.
Why Metals Are Solids
Except for mercury (and possibly copernicium and flerovium), elements that are metals are solid at room temperature. Francium, cesium, gallium, and rubidium melt into liquids at temperatures slightly warmer than room temperature. Metals tend to have high melting points because their atoms form metallic bonds with one another. Essentially, metal atoms share electrons, forming a sea of negatively-charged electrons between positively-charged nuclei.
Why Mercury Is a Liquid
Mercury has a low melting point and is a liquid at ordinary temperatures because its electrons aren’t readily shared between its atoms. This is a consequence of mercury atoms containing so many protons and electrons and the way its electrons organize around the nucleus.
Atoms containing a large number of protons are relatively small because the large positive electrical charge exerts a strong attraction over the electrons. This is a periodic table trend that partially explains differences between the melting points of elements.
What makes mercury special is its electron configuration: [Kr] 4d10 4f14 5s2 5p6 5d10 6s2
The filled 4f shell poorly shields valence electrons from the positive nuclear charge. The 6s electrons draw close to the atomic nucleus, shrinking the atomic radius. Orbiting such a large nucleus means the electrons move at relativistic speeds and act much more massive. Relativistic effects account for about 10% of lanthanide contraction. Yet, the lanthanides are solid metals.
Unlike these elements, mercury atoms have a filled 6s shell. The highly stable valence shell means atoms don’t easily gain or lose electrons. Coupled with the strong attraction between the valence electrons and the nucleus, mercury acts like a noble gas. Its atoms just don’t interact with one another strongly enough to solidify at room temperature.
Other Mercury Properties
Because mercury isn’t good at sharing its electrons with other mercury atoms, it doesn’t conduct heat or electricity as well as other metals. This is also why solid mercury is a soft metal. Mercury doesn’t readily form chemical bonds with itself and is the only metal that does not form diatomic molecules (Hg2) as a gas.
Why Gold and Thallium Aren’t Liquids
Like mercury, gold and thallium atoms have low energy 6s electron orbitals. Atoms of all three elements have massive nuclei, experience relativistic effects, and have filled 4f shells. But, both gold and thallium are (soft) solids at room temperature. Why? The answer lies in the electron configuration of these metals.
| Element | Atomic Mass | Electron Configuration |
|---|---|---|
| Gold (Au) | 196.9665 | [Kr] 4d10 4f14 5s2 5p6 5d10 6s1 |
| Mercury (Hg) | 200.59 | [Kr] 4d10 4f14 5s2 5p6 5d10 6s2 |
| Thallium (Tl) | 204.383 | [Kr] 4d10 4f14 5s2 5p6 5d10 6s2 6p1 |
The gold 6s orbital is only half-filled. So, even though though the 6s electron is tightly bound, a gold atom readily accepts another electron and participates in metal-metal bonding. Gold is a relatively inert noble metal because it does not easily yield its valence electron.
A thallium atom is even more massive than a mercury atom. It has a filled 6s orbital. But, it has a lone 6p electron. This electron can’t get as close to the nucleus as the 6s electrons. It’s fairly reactive, so it participates in metallic bonding and commonly forms the Tl+ ion.
References
- Cotton, F. Albert; Wilkinson, Geoffrey (1988). Advanced Inorganic Chemistry (5th ed.). New York: Wiley-Interscience,. ISBN 0-471-84997-9.
- Housecroft, C. E.; Sharpe, A. G. (2004). Inorganic Chemistry (2nd ed.). Prentice Hall. ISBN 978-0-13-039913-7.
- Lide, D. R., ed. (2005). CRC Handbook of Chemistry and Physics (86th ed.). Boca Raton (FL): CRC Press. ISBN 0-8493-0486-5.
- Norrby, L.J. (1991). “Why is mercury liquid? Or, why do relativistic effects not get into chemistry textbooks?” J. Chem. Educ. 68(2): 110. doi:10.1021/ed068p110
- Rustad, D. S. (1987). “How soft is mercury? (Letter to the Editor)”. J. Chem. Educ. 64:470.