19.5 Why is Mercury a liquid at room temperature?
Description
This article is from the Chemistry FAQ, by Bruce Hamilton B.Hamilton@irl.cri.nz with numerous contributions by others.
19.5 Why is Mercury a liquid at room temperature?
First, let's look at the melting points of some of the elements surrounding
mercury in the periodic table ( in degrees C ) :-
Period IB IIB IIIA 4s3d4p Cu 1083 Zn 419.5 Ga 29.8 5s4d5p Ag 960.8 Cd 320.9 In 157 6s(4f)5d6p Au 1063 Hg -38.4 Tl 304
The interesting comparison is between Hg and Au, as their properties differ
dramatically, although their electron structures are similar:-
14 10 1
Au(g) : Xe | 4f , 5d , 6s
79 54
14 10 2
Hg(g) : Xe | 4f , 5d , 6s
80 54
Very few chemistry textbooks discuss relativistic effects on chemical
properties, despite the availability of a comprehensive review by P.Pyykko
[12]. There several good introductory articles on the derivation and
calculation of various relativistic effects in molecules and atoms, so I'm
not going to include details [13,14,15]. Suffice to say, that whilst
smaller elements can treated simply, larger elements need treatment based
on the Dirac equation, which shows that the s electrons are approaching
the speed of light, consequently relativistic effects are important.
If we take the relativistic mass of mercury (m);-
Mo where
m = -------------------- c (speed of light) = ~137 atomic units
_____________ v = Z = 80
/ ( v ) 2 Mo = rest mass
/ 1 - ( - )
\/ ( c )
The masses of the 1s electrons are increased by approximately 20% over
their rest masses, which means that the radius is decreased by 20% - since
mass appears in the denominator of Bohr radius calculations. All the other
s shells also contract, with the 6s contracting ~14%, because their electron
speeds near the nucleus are comparable, and the contraction of the inner part
of the wave function also pulls in the outer tails. The p orbitals also
contract a similar amount, and these contractions also results in increases
the screening for d and f orbitals, which may then expand - about 3% for the
5d orbital of mercury.
In mercury, the relativistically-contracted 6s2 orbital is full, thus the
the two electrons do not contribute much to the metal-metal bond, which is
not the situation for gold. The bonding in mercury is believed to be mainly
van der Waals forces with a contribution from 6p orbital interaction. The
relativistic contraction of the filled 6s2 orbital, when added to the
contraction across the sixth row of the periodic table, results in relatively
weak Hg-Hg bonds that are responsible for mercury being a liquid at room
temperature. For those curious to know more, a recent article in J.Chem.Ed.
provides much more detail and several good references [16]. Relativistic
effects are also responsible for the colour of gold ( partially explained
by the 5d -> 6s transition in gold requiring less energy than the 4d -> 5s
transition in silver, resulting in a smaller d-s gap ) [12,14,16].
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