Why is Hg a liquid at room temp?
Name: t hammargren
Why is mercury a liquid at room temperature although elements
surrounding Hg in the periodic chart are solids?
An excellent question! It is a little odd...after all,
it's right next to gold, silver, cadmium, indium and thallium...
all of which are solids at room temperature.
So why is this one physical property apparently so very different?
The first thing to note is that all of these neighboring metals
are ductile and malleable...they're soft, in other words.
Mercury, as a liquid metal, is extra-soft and extra-malleable...
so now we see that the difference between the two is not
as extreme as we thought. Note that liquid mercury has a pretty
high surface tension too...that means that the cohesive forces
between atoms must be reasonably strong. Yet another example that
mercury's fluid nature is just an extreme of the behavior of
its soft neighbor metals. Now, consider the fact that all the
neighboring metals have (on the scale of things) relatively low
melting temperatures...if "room temperature" were just a bit
higher, some of the neighbors would be liquids too. For example,
solid cadmium may be cut with a butter knife, and melts at 321 K...
just 23 K above room temperature.
If this response does not completely answer your question, I hope
that it provides you with food for thought.
The best comparison is with elements in the same column of the periodic
table, which one would expect to have very similar properties, that being
one meaning of the table. Reading from top to bottom we see a downward
trend in melting points from Zn (420 C) through Cd (321 C) to Hg (-39 C),
and, as Dr. Topper points out, Hg doesn't look quite so much like a
stand-out. Why does the melting point decline? A solid melts when the
available energy in heat exceeds the energy required to break up the solid,
which is equal to the extra energy gained by letting the atoms stay always
close to one another rather than passing only occasionally near like ships
in the night. The simplest model of metals treats the valence electrons as
a gas around charged cores composed of the atoms minus their valence
electrons. Zn, Cd and Hg all have the same number of valence electrons (2)
and hence the same charge on their cores, *but* the volume each valence
electron has to move around in increases from 8 to 11 to 12 in units of
10^-24 cm^3, that is the gas of valence electrons becomes less dense. When
it is less dense, the electrons interact (favorably) with the oppositely
charged cores less often, so the extra energy gained by having cores and
electrons all close together is less, hence the extra energy of the solid
is less, hence the melting point lower. Why should the valence electron
gas get less dense? Not an easy question to answer, the density is a
delicate competition between the desire of all the electrons to get close
to the cores and their mutual repulsion. Since the Hg core is bigger (it
has more non-valence electrons in it) the electrons are squeezed anyway in
Hg by the core hogging more space, so you might guess the preferred density
might end up a little lower, as it does.
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Update: June 2012