THE MODERN EXPLANATION OF CRYSTAL
SHAPE
The hexagonal symmetry of an ice crystal is
an outward manifestation of an internal arrangement of the atoms in the ice.
Ice is composed of water, with the formula:
H2O, meaning there are two lighter
hydrogen atoms and one larger oxygen atom bonded together.
Cooling of a substance means that the active
molecules slow down and do not move as fast.
When water molecules are heated, molecules
do not associate readily and become steam.
At normal, or room temperatures, the water
molecules tend to cling together tightly enough that something called surface
tension is manifested. Surface tension can cause objects like pieces of paper
to float on top the water without absorbing any. Small specialized spiders
called water skippers use surface tension to support themselves on water and
navigate without sinking.
As water cools ice crystals form. The
molecules slow down enough that a lattice of molecules is formed. This lattice
tends to keep the molecules together and somewhat bond in more or less regular
shapes. Molecules vibrate, but as the water molecules cool, they tend to
vibrate in a particular alignment. One would expect that because the molecule
is three pronged, that ice crystal lattices would align in patterns of three.
That is almost correct. Generally speaking ice crystals form six sided
structures.
Each ice molecule is V-shaped with an angle
of 109 degrees between the legs:
Ice molecules are bound together in an open
lattice and form puckered layers with hexagonal symmetry. Each molecule is
surrounded by four nearest neighbors, so that each group has one molecule at
the center and the other four at the corners of a tetrahedron, all at the same
distance away. The molecules are held in place mainly by electrostatic
attraction between the positive charge of the hydrogen atom and the negative
electrons of the neighboring oxygen atom. This is called a hydrogen bond.
Ice crystals grow as thin hexagonal plates
or long hexagonal columns, depending on temperature. Two faces can be defined
for ice crystals--the basal face and the prism face. The basal face if
typically the surface which shows hexagonal symmetry. For example, the basal
face above is the surface facing the reader. Both the upper and lower surfaces
are basal faces. The prism face is perpendicular to the basal face. It faces
outward from an arm or portion of an arm. This face does not exhibit hexagonal
symmetry. For some temperatures, the basal face grows faster than the prism
face, resulting in long hexagonal columns or needles. At other temperatures,
the prism face grows faster, resulting in thin hexagonal plates, fern-like
stellar crystals, and dendrites like that shown above. The fern-like dendritic
nature of crystals is caused by the humidity. The greater the humidity, the
more feathery the crystals will appear.
Hallet and Mason have explored the reasons
the basal and prism faces grow at different rates as a function of
temperature. They have discovered that vapor molecules collect on the surfaces
and migrate across the surfaces to their final lattice positions. The rate at
which the molecules migrate across the surface varies with temperature, and is
different for the basal and prism faces. For some temperature ranges there is
a net surface migration from the basal to the prism faces, resulting in a
plate-like shape or habit. For other temperature ranges the situation is
reversed, resulting in a net flux of molecules from the prism to the basal
faces and the formation of columns or needles.
Here is a pattern of how the ice crystals form at the atomic level, with
the blue balls representing the hydrogen, and the red balls representing the
oxygen:
Snowflake Gallery
