Chemistry Reference
In-Depth Information
a triangular pattern can be found in the direction of every of the four body
diagonals. These layers can be shifted easily in four directions when forces have
an impact on the metal crystals. The hexagonal packing only has one such shift
direction; therefore, metals with a hexagonal structure like magnesium and zinc are
not very ductile. Metals with a cubic face-centered structure like gold, silver,
copper, or
-iron can be rolled out to thin wire or very thin sheets (“gold leaf”).
The shape memory effect of memory metals like “Nitinol” (NiTi), a special alloy
of nickel and titanium, is also based on a diffusionless and therefore lightning-fast
change of structure at certain temperatures. But the memory effect cannot be
explained without the formation and translation of twin crystals [
8
,
12
].
g
It is possible (but not usual) to give information about the different structures of
metal crystals with the help of structural symbols. The written symbols state that a
three-dimensional infinite packing of metal particles is given: Mg atoms form a
hexagonal packing with the coordination number 12, while Fe atoms form a cubic
packing with the coordination number 8. These symbols are called Parth
´
symbols.
Other symbols show the coordination number in curly brackets and indicate that a
three-dimensional lattice is given: Niggli-symbols.
10.5.2 Metal Reactions: Regrouping of Metal Atoms
Sodium and mercury react vigorously when a small piece of sodium is crushed in a
drop of mercury [
13
]. This reaction can be understood as a regrouping of Na atoms
and Hg atoms into a new metal lattice with both kinds of atoms distributed
statistically (see Fig.
10.12
).
+
Sodium (s)
Mercury (l)
Sodium amalgam (s);
Δ
H < 0
Fig. 10.12 Model drawing for the regrouping of metal atoms by forming sodium amalgam [
13
]
