Why Is Mercury a Liquid?

Mercury is the only metal that is a liquid at normal temperatures and pressure. Why is mercury a liquid? What makes this element so special? Basically, it's because mercury is bad at sharing—electrons, that is.

Most metal atoms readily share valence electrons with other atoms. The electrons in a mercury atom are bound more tightly than usual to the nucleus. In fact, the s electrons are moving so fast and close to the nucleus that they exhibit relativistic effects, behaving as if they were more massive than slower-moving electrons. It takes very little heat to overcome the weak binding between mercury atoms. Because of the behavior of the valence electrons, mercury has a low melting point, is a poor electrical and thermal conductor, and doesn't form diatomic mercury molecules in the gas phase.

The only other element on the periodic table that is a liquid at room temperature and pressure is the halogen bromine. While mercury is the only liquid metal at room temperature, the elements gallium, cesium, and rubidium melt under slightly warmer conditions. If scientists ever synthesize a sufficient quantity of flerovium and copernicium, these elements are expected to have an even lower boiling point (and perhaps melting point) than mercury.

Source

• Norrby, L.J. "Why is mercury liquid? Or, why do relativistic effects not get into chemistry textbooks?" Journal of Chemical Education.