The structure of
crystals. The
crystalline state
It
is well known that matter is formed by atoms (and/or ions) that
sometimes group together to form what we know as molecules.
Historically, these aggregates (atoms, ions or molecules) have been
classified in the three fundamental states of matter: gas, liquid and
solid. The so-called crystalline state corresponds fundamentally to the
solid state, but with very special characteristics.
Matter is considered strictly
crystalline if the atoms, ions or
molecules that compose it are distributed in the three independent
directions of space in a regular and repetitive way. This form of
matter is called crystal,
a word
(κρύσταλλος,
crustallos,
or phonetically Kroos'-tal-los
= cold + drop) that ancient Greeks identified with the mineral quartz,
defining it as icicles of extraordinary hardness and very cold.
Crystals usually show their internal order by the appearance of regular
external morphologies, that is, through perceptible faces and edges, as
it is seen in the two samples shown below.
Crystals of quartz and boleite (a complex
halide mineral)
A crystalline packing of molecules,
showing its highly ordered structure that extends in all
directions
Although the concept
of crystal is usually associated with the solid
state of minerals, biological molecules such as proteins give rise to
extremely fragile crystals, with water contents reaching up to 80% of
the crystal volume, representing a state much closer to the liquid than
to the solid. However, they generally show an external morphology that
reveals their internal order.
Protein
crystals. Up of 80% of their volume is water!!!
In another sense the
crystalline state can also be considered as a
limit of the evolution of the liquid state towards solid. In the liquid
state the molecules, very close to each other, come into contact and
retain a certain attraction with their neighbors, and hence when we
transfer a liquid from one container to another, it maintains a
constant volume. However, in part due to the thermal agitation,
intermolecular contacts are not stable enough to maintain the volume of
the liquid rigid. If we reduce the thermal agitation the fragile
contacts (or bonds) between molecules will become more and more stable,
reaching a relatively rigid state. If the temperature decrease has been
slow enough, the molecules pack together in an orderly manner that
corresponds to the most stable possible situation, the one with the
lowest energy. This is the crystalline state.
The water
molecules in the liquid state attract each other through a special type
of dipole-dipole interaction known as hydrogen bonding. The molecules
undergo rapid thermal motions, so the lifetime of any specific
clustered configuration is very short
The ordered crystal structure of ice. Its
three-dimensional structure is maintained by stable hydrogen bonds
But let's
go back...
