Chemistry Reference
In-Depth Information
taken and their arrangement in crystals can be discussed by comparing the sugar
crystals of the same shape (“rock sugar”).
Symbols on the level of Dalton's atomic model.
Dalton's philosophy, published in
1808, includes the connection of the term “element” with the idea of the “atom”: there
exist as many different kinds of atoms as there are elements, the atoms of different
elements differ in their masses. First atomic mass tables were developed and first
atomic symbols used (see Fig. 5.1). Dalton led the search of the Greek philosophers for
the basic units of matter to first results with his ideas. He suggested new compounds
through a combination of atoms; unfortunately he assigned the molecular symbol
HO to the compound water resulting in the mass of 1 + 8 in first atomic mass
tables (instead of 2 + 16). Avogadro's hypothesis together with the experimental
results of Gay-Lussac led to the symbol H
2
O for a water molecule (see Fig.
7.6
).
In the easiest case combinations of metal atoms of one kind form a closest
packing: balls can be piled up as packing of spheres with the coordination number
12 (see Chaps. 6 and 10). It is also possible to substitute metal atoms in a metal
structure by other metal atoms: crystals of alloys can be described in this way. They
can be illustrated with packing of similar-sized balls and different colors (see
details on models of metal structures and alloys in Chap. 10).
Combinations of nonmetal atoms mostly lead to molecules: H
2
,O
2
,Cl
2
,P
4
,S
8
,
for elements, and H
2
O, HCl, H
2
SO
4
,P
4
O
10
,orH
3
PO
4
in compounds. In molecular
model kits the bonding number of nonmetal atoms is usually represented by push
buttons or similar on the balls. They can be used to build molecular models in the
form of ball-and-stick or calotte models or to demonstrate the regrouping of atoms
in chemical reactions (see Fig.
7.6
). Many reactions of organic school chemistry,
similar to the previous example, can be illustrated with molecular models,
visualized with drawings and symbolized with structural symbols.
Ions would have to be added to the atoms to complete the “kit of the basic units
of matter” according to current knowledge. The students get an illustration of
important atoms and ions in the form of a periodic system as shown in Fig.
7.7
water vapor(g);
Δ
H
Hydrogen (g)
+
Oxygen (g)
< 0
molecules
molecules
molecules
Fig. 7.6 Mental model for regrouping atoms in molecules
