Chemistry Reference
In-Depth Information
Instruction with structural models equals modern structure analysis: in all cases
results lead to smallest structural units of substances. For molecular substances the
molecule represents such a unit and the chemical formula is stated for the molecule.
The symbols for acetic acid or the benzene molecule are CH
3
COOH and C
6
H
6,
respectively. These symbols would not be shortened to CH
2
O and C
1
H
1
or CH,
respectively. This agreement should be transferred to symbols for ionic solids. The
smallest structural unit should also be symbolized there - the unit cell. Symbols for
the unit cell of sodium chloride should be {(Na
+
)
4
(Cl
)
4
}orNa
4
Cl
4
(see Fig. 6.3).
The teaching process may start with demonstrating models of unit cells and
counting of ions (see Chaps. 6 and 10). Symbols like Na
4
Cl
4,
Li
8
O
4
or Zn
4
S
4
should be derived for the smallest structural units - similar to the symbols
CH
3
COOH or C
6
H
6
for molecules as smallest structural units of corresponding
substances.
In the second step the symbols can be abbreviated to the most common symbols -
but these lack information: NaCl, Li
2
O or ZnS. In this way students learn about
the importance of formulae and their connection to chemical structure and gain
a modern concept of chemistry. Tests on spatial ability have shown that the
majority of middle school students is able to spatially identify unit cells drawn in
two dimensions [
17
] and to count the number of ions successfully [
18
].
7.3 Learner-Student Concepts of Structures and Symbols
Every student knows abbreviations like PC or MP3. Therefore students might think
that chemical symbols are similar formal abbreviations of substance names:
examples like NaCl, CaO or MgO support this concept. If students do not have
a structural concept, they are not able to understand the subscripts in symbols like
H
2
OorAl
2
O
3
and just learn them by heart. They keep their incorrect or contradic-
tory concepts and see the chemical symbols as some kind of secret language.
Concepts of combustion.
In Chap. 1 we introduced a student's mental model for
the combustion of magnesium as an example. The student formulated the correct
reaction symbol “2 Mg + O
2
!
2MgO,” but his real concept was: “Magnesium
consists of two kinds of particles. One vaporizes during combustion, the other
remains as magnesium oxide.” He also made a drawing accordingly (see
Fig. 1.6). Empirical research with about 300 grade 9 and 10 students found that
almost all of them wrote down the correct reaction equation, but 70% of them had
inadequate mental models and produced wrong drawings [
19
]. It is evident that
mental models from everyday life, which students construct by observation over
many years, cannot be transferred to scientific concepts just by the formulation of
reaction symbols. Only models of the structure of substances before and after
combustion make the reaction equation understandable and can support the under-
standing of combustion processes [
14
].
Concepts of the term “ion.”
The following test was conducted with a group of
high school students in grade 10 that already knew the terms “ion” and “electron
