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Why mercury is liquid at room temperture?

  1. Nov 30, 2006 #1
    Mecury has a boiling point of -39 centigrade but is relatively dense with 80 protons. It has 2 valence electrons like all the other transition metals but why does it have the strange property of being a liquid at room temperture?
  2. jcsd
  3. Nov 30, 2006 #2
    Why wouldn't it?

    Start of by defining a solid and a gas and how the transformation from a solid to a gas takes place. The energy needed to break the intermolecular forces in solid is proportional against the force holding atoms together. The less of this 'binding energy', the less energy it takes to break the solid structure and turn it to gas and as a result, the boiling point is lower. Although this is a pretty trivial description, it should give you the basics of it.

    Do not assume that room temperature has some specific relevance because it doesn't really mean anything.
  4. Dec 1, 2006 #3
    I understand that. The question is why does mecury have such low binding energy relative to most of the other metals? This raises the question how does transition metals bind among themselves?

    Wiki has Hg having 1 or 2 delocalised electrons so with its large neclues tends to block the electrostatic attraction but Thallium has one more proton and has mostly 1 delocalised electron but it has a boiling temperture of 304 centigrade. Why is this?
    Last edited: Dec 1, 2006
  5. Dec 1, 2006 #4


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    1. The 6s2 configuration gives Hg a fully filled valence subshell, with all inner shells completely filled (you don't see this in any other metals besides Zn and Cd). This makes Hg pretty inert.

    2. In addition, the binding energy between atoms in Hg is low because the valence electrons (the 6s pair) are very tighly bound to the nucleus. One reason for this is that the nuclear charge in the Hg nucleus is very poorly screened by the 4f electrons, which are very diffuse spatially. This allows the 6s electrons to see a higher effective charge than in the case of Zn or Cd (where the screening is provided mostly by electrons in d-subshells which are a little better than the f-electrons at screening) and hence be more tightly bound. The other "reason" (one I'm not too fond of), is that Hg, being a big atom, the valence 6s electrons have a large momentum, and as a result, the relativistic correction to their energy puts them in a slightly lower energy state than that which would be naively obtained without considering the correction.

    This tighter binding to the nucleus making the 6s energy small is also responsible for the relative inertness of the Hg atom.

    PS: Note that Zn and Cd also have pretty low melting points (~ 300C, 400C) compared to the typical number (1000-2000C) for transition metals.
    Last edited: Dec 1, 2006
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