Mercury

Mercury is liquid at 25 °C for the same relativistic reason gold is yellow: the 6s pair is contracted and held so tightly that mercury atoms barely share electrons with each other, leaving a metal whose atoms are bound about as weakly as a noble gas's, melting at −39 °C. That liquidity plus a uniform expansion coefficient made it the fluid of thermometry and of Torricelli's barometer, which is why atmospheric pressure is still quoted in millimetres of mercury. It also amalgamates gold, the basis of both a two-thousand-year-old refining technique and today's most damaging artisanal-mining pollution. The Minamata Convention, named for the Japanese bay where methylmercury discharge poisoned thousands from the 1950s, now phases mercury out of most products worldwide.