Chemistry Reference
In-Depth Information
atomic masses leads to the atomic ratio: most young students are overchallenged by
this method.
Kaminski and Jansen [
12
] therefore propose the number of atoms in 1 mg of
substance to calculate formulae:
for example 1 mg of carbon contains of
10
17
O atoms. After
having the numbers of reacting atoms the calculation of formulae is simplified: if
laboratory measurements show that 10 mg of carbon react with 26.7 mg of oxygen
the numbers of 5,020
10
17
C atoms and 10,040
10
17
C atoms, 1 mg of oxygen contains of 376
502
10
17
O atoms can be calcu-
lated very easy and the CO
2
symbol derived. But one problem remains: naturally
the numbers are of the order of 10
17
and young students are not used to work with
this kind of big numbers.
Historically analytical balances were used to deduce chemical symbols.
Corresponding analysis methods have been developed by Liebig and Berzelius in
the mid-nineteenth century and were in use for a long time. Methods of instrumental
analysis became available in many chemical institutes step by step from the 1960s:
spectral analysis, gas chromatography, X-ray structure analysis, atom absorption
spectroscopy, UV-, IR-, NMR-spectroscopy and mass spectrometry. Today
analyses of substances are carried out with these instrumental analysis methods.
Deduction of symbols from structural models.
Experts are able to identify the
structure of crystalline substances with the help of X-ray structure analysis. Lattice
constants as well as bond angles and bond lengths can be determined using
dedicated software; even spatial illustrations of the chemical structure can be
printed out.
The concept of X-ray structure analysis should be interesting for students if some
exemplary Laue diagrams are shown (see Fig. 5.3) with the corresponding diffrac-
tion lattices being illustrated with laser beam experiments [
13
]. If students are given
either a structural model of a molecule or a unit cell of a crystalline structure, they
are able to deduce the empirical formulae from the models by counting the ratio of
atoms or ions in these (see Chaps. 6 and 10). This instructional method fits the
educational demand to add the level of structural models to facilitate the under-
standing of chemical symbols [
14
]. This is schematically described in Fig. 3.8:
phenomena (substances and reactions) are introduced first, then corresponding
structural models, and finally derived formulae and chemical equations.
This sequence is in tune with Johnstone's “Chemical Triangle” [
15
], concerning
the connections of substances, structural models and chemical symbols (see
Fig.
7.9
): the Macro level shows substances and reactions and all that “can be
seen, touched and smelled,” the submicro level contains all considerations of
involved atoms, ions, molecules and chemical structures, while the representational
level expresses symbols, formulae, equations, calculations, tables and graphs. In
this sequence the introduction of chemical formulae should be possible.
Johnstone also warns that mixing these levels has to be done with care: “It is
psychological folly to introduce learners to ideas at all three levels simultaneously.
Herein lies the origin of many misconceptions. Trained chemists can keep these
three levels in balance - but not the learner” [
15
]. Gabel points out that a big
mistake in chemical instruction seems to be the jump from the macro level direct to
