Chemistry Reference
In-Depth Information
10.3
Ions on the Level of Dalton's Atomic Model
The big class of salts cannot be integrated easily into structure-oriented chemistry
lessons, because ions - the smallest particles of salts - do not exist in Dalton's
atomic model. The introduction of the term “ion” calls for the differentiated atomic
model and we shall illustrate this concern with one example.
During the electrolysis of salt solutions or molten salts, deposition of substances
at the electrodes can be observed. The decomposition is explained with the trans-
formation of ions to atoms transferring specific numbers of electrons. The charges
of ions can be derived by comparing the number of protons in the nucleus and the
number of electrons in the shell. Differences of properties between salts and
involved elements are described by differences in the atomic and ionic structure.
The downside of this approach is the fact that the term “ion” cannot be used in
chemistry lessons before the introduction of the differentiated atomic model. Until
then, the smallest particles of salts are not called “ions,” but “imaginary molecules,
chemical units, units of compounds, compound particles, or even atoms.” [
3
]This
circumscription of the term “ion” leads to vague concepts, in most cases to the well-
known molecule concepts: empirical studies show that even high school students in
grade 11 or 12 after several years of chemistry lessons still think, that salts and salt
solutions are built of molecules [
4
].
It is possible to introduce ions earlier than usual, e.g., with the freezing point
depression [
5
]. Initially, it has to be noted that the freezing point depression rises
proportionally with the increase of the concentration of dissolved particles. The
freezing point depression of, e.g., 1-M ethanol solution or 1-M glucose solution is
1.9
C.
However, the freezing point depression of a 1-mol sodium chloride solution is
5.7
C[
5
]. It can be derived
from these measurements that 1 l of a sodium chloride solution contains 2 mol
dissolved particles, 1 l of calcium chloride solution contains 3 mol dissolved
particles. If the fact is known that salt solutions conduct electric current and
explained with dissolved particles that carry electric charges, the name “ion” can
be introduced and combined with the following conclusions from freezing point
depressions of 1-M solutions [
5
]:
3.8
C, and of 1-mol calcium chloride solution it is
1mol Na
þ
(aq) ions
1mol Cl
(aq) ions
1mol NaCl
!
þ
¼
2 mol ions per litre
1molCa
2
þ
ð
2molCl
(aq) ions
1molCaCl
2
!
aq
Þ
ions
þ
...¼
3mol ionsper litre
If the ions are thought of as smallest particles carrying electric charges, they can
be identified by ionic symbols and like atoms introduced as basic particles of matter
according to Dalton's model . They can be visualized by spheres with specific
diameters and integrated into the Periodic Table of the Elements (PSE) as smallest
particles of salts, as Christen suggests [
6
]. Sauermann and Barke [
7
] adopted this
kind of periodic table (see Fig. 10.4, and
www.thinking-chemistry.org
).
