Thinking back to my undergraduate years in Chemistry 101, it always struck me as strange that mercury, a transition metal, was liquid at room temperature. When I asked the professor about it, he said the explanation was by no means simple and involved both the electronic configuration of mercury and relativity as it related to the outermost electrons of that element. Seems they orbit at such a significant fraction of c that they weigh more as a consequence. But how does this make mercury a liquid at STP? I still cant recall the explanation.