778. Why is the element mercury a liquid at room temperature when none of its neighbors on the periodic table are? — BZ, Trenton, NJ
The answer to that question lies at least partly in the electronic structure of the mercury atom. The mercury atom is the largest member of the third row of transition metals, meaning that it is the atom at which the 5d shell of electrons is finally filled completely. Whenever a shell of electrons is filled, that shell can no longer assist in forming chemical bonds. While the d shell electrons normally help hold transition metal atoms together, making these metals strong and hard to melt, the filling of the 5d shell makes it hard for mercury atoms to stick to one another. In contrast to metals like tungsten and tantalum, which melt only at very high temperatures, mercury is a liquid at room temperature. Actually, the zinc atom is the atom at which the 3d shell is filled and the cadmium atom is the atom at which the 4d shell is filled. While those two metals are solid at room temperature, they have very low melting points.