Mercury has a incredibly low melting point - unlike most metals(Tungsten: 3,422C, Iron: 1,538C). Its melting point is about -38.83 in celcius, even lower than that of water. Then, what makes this matter so special? Why is it so easily melted, even though it's a metal? The answer is: the bonding force between Mercury atoms is too weak.
Metals can make only a 100 kinds of bonds, when there are more than 50 thousand types of bonds in the world. It is because metals have a strange way of bonding. Unlike ionic bonds or covalent bonds, metalic bonds can only happen between same types of metals. In a metalic bond, the metal atoms are bonded together by free electrons, that can roam around nuclei easily. These free electrons give metals the characteristics of malleability ductility. The more free electrons a bond has, the stronger it becomes.
In the case of Mercury, however, the bonding force between the atoms are too weak to from a soild in high temperatures. Here's why. The orbital of Mercury is 1s2 2s2 2p6 3p6 3d10 4s2 2p6 4d10 5s2 5p6 4f14 5d10 6s2. Here, the out most layer of electrons are in the 6s2 orbital. But when realigned into a way ordered in the energy of the orbitals, Mercury's electrons are formed like [Xe] 6s2 4f14 5d10. In both ways, it can be shown that mercury is a very stable matter, and thus not forming bonds well. Also, because the outermost electrons are in the 6s2 orbital, thier energy levels are low, and are more closely attached to the nucleus than other orbitals(d,f). As a result, the effective nuclear charge of the 6s2 orbital in a Mercury atom is stronger than the attractive forces of nearby atoms. This makes it even harder for Mercury metalic bonds to have enough free electrons to form a stable bond. In addition, due to the van der waals force, the nuclei of Mercury atoms push each other away, too.
In a similar way, Galium([Ar] 3d10 4s2 4p1), Rubidium([Kr] 5s1), and Ceisium([Xe] 6s1) melt in a low temperature, just above room temperature(25C). It is also believed that Copernicium([n 5f1 6d10 7s2) and Flerovium([Rn] 5f14 6d10 7s2 7p2) will have an even lower boiling and melting temperature than those of Mercury. These matter described in this paragraph have valence electrons that only fill in a s orbital or a part of the p orbital. This makes the atoms less likely to share an electron.
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