Chemistry Reference
In-Depth Information
a
b
c
Cl
-
-Ion
Na
+
-Ion
Fig. 6.3 NaCl-crystal structure:
ball-and-stick
model, cubic sphere packing and unit cell
w'
O
X
X'
M
X
Y'
Z
?
S
O; Original
S; Student
M; Model
Fig. 6.4 Scheme of the way of perception by models, according to Kircher [
4
]
The process of perception through models is also described by Kircher [
4
]. His
concept will be explained, using sodium chloride crystals and corresponding
models (see Figs.
6.3
and
6.4
):
- The original O is to be a natural rock salt crystal with a cubic shape, planar
surfaces, straight edges and right angles
- The packing of spheres is to be chosen as the model M (see Fig.
6.3b
). The
chloride ions are represented by big balls and the sodium ions are represented by
small balls
- The student S can now understand the original O with the help of the model M,
the sphere-packing model works as a mediator between the student S and the
original O
1. There are properties
x
and
y
of the crystal that have corresponding model
characteristics
x
0
and
y
0
. For example,
x
is chosen for the spatial arrangement of
sodium and chloride ions in the crystal,
x
0
is then chosen for the corresponding
arrangement of big and small balls with the coordination number 6 in the model.
If
y
depicts the radius ratio of both types of ions in the sodium chloride crystal, then
y
0
depicts the corresponding size ratio of the balls in the sphere-packing model. So
x
and
y
represent the parameters in the original,
x
0
and
y
0
those which are displayed
in the model. According to Stachowiak
x
and
y
are the features of depiction,
according to Steinbuch these are “the essentials, which are let through the filter.”
2. There exist characteristics
z
in the original that do not have an analogy in the
model. The salty taste or the white color of the crystal, for example, are not
