Chemistry Reference
In-Depth Information
6.1 Scientific Ideas: Thinking in Models
Chemistry found recognition and success, when it departed from trial and error
approach of medieval alchemy. Starting with simple laboratory experiments for the
description of substances it came to develop first mental models for the structure of
matter in the eighteenth and nineteenth century. Some states of perception will be
given exemplarily.
Dalton postulated in 1808 that there are as many kinds of atoms as there are
known elements. He presented the first atomic mass table, which has been corrected
and extended mainly by Berzelius in the next decade. The comparison of experi-
mentally determined mass ratios of elements in compounds with their atomic
masses made the empirical analysis possible and led to the knowledge of the
composition of many substances as well as the first empirical formulae.
Kekul
´
deduced the theory of valence from his experience in 1865: He created a
first concept of molecular structure with the four-bonding or the tetrahedral model
of the carbon atom, the one-bonding model of the hydrogen atom, and the two-
bonding model of the oxygen atom. With these mental models it became possible to
predict structures of many molecules, verify them in experiments and plan targeted
syntheses of new substances.
In 1912 Laue discovered three-dimensional crystal structures through the dif-
fraction of X-ray radiation from salt crystals: he obtained X-ray diffraction patterns,
calculated those three-dimensional interference patterns and deduced the structure
of investigated salt crystals. Based on these results Bragg solved the crystal
structure of sodium chloride in 1914. This destroyed the historical idea of NaCl
molecules in table salt: sodium ions and chloride ions with the coordination number
6 form the salt crystals. All following structural analyses were based on Laue's and
Bragg's fundamental discoveries, providing the chemical structure of many crys-
talline substances, in turn making the syntheses of new substances possible.
Models and scientific perception
. The idea of models and the process of scien-
tific perception will be explained with a diagram of Steinbuch [
3
] (see Fig.
6.1
):
“Any facts of reality, an original, can be transferred to an abstract mental model by
taking only the essential and relevant parts of the given context. Certain informa-
tion, for example generally accepted laws of logic or physics may be added.
A mental model for future thinking processes is thereby available. For the purpose
of illustration, this abstract mental model can be transferred back into reality by
building a concrete display model or even by artistic representation. But these have
unavoidable irrelevant components, which the mental model does not have.”
This “thinking in models” can be transferred to Laue's way of perception
(see Fig.
6.2
): The interference pattern (original) that forms from a salt single-
crystal in the Laue experiment is let through the filter as “essential.” Interfer-
ences of light with two-dimensional diffraction gratings and their calculations are
familiar (additional information). They are taken as a basis for the calculations of
three-dimensional diffraction gratings. The result is a mental model for the three-
dimensional symmetrical structure of salt crystals built of ions. The ions in the ionic
