Chemistry Reference
In-Depth Information
displayed in the model. The model is shortened by this characteristic: Stachowiak
calls this “feature of shortening.” Similarly the density or melting points of
crystals cannot be determined with models. The model builder, however, never
had the intention to transfer these original characteristics to the model.
3. There may be characteristics
w
0
in the model that do not have an analogy in the
original. The choice of colors - for example white for the big balls and red for the
small balls - represents a model characteristic, which is totally irrelevant and
arbitrarily chosen by the model builder: Steinbuch calls these irrelevant components.
Additional irrelevant components are model materials like wood, cellulose or
styrofoam and an adhesive between the balls, such as glue or Velcro tape.
The crystal structure model (see Fig.
6.3a
) only shows the cubic arrangement of
ions in the crystals, not their size. The option of looking into the model, however,
allows a better illustration of the coordination number 6. The connecting sticks
between the junctions are only necessary for the stability of the model, they are totally
irrelevant components towards the original and do not have a representation function.
The unit cell model (see Fig.
6.3c
) shows three features of depiction: the cubic
structure of the ionic compound, the size ratio of the ions and the correct 1:1 ratio of
the number of ions. If all parts of the big and small balls of the model are summed
up, it results in four big and four small balls. Transferred to the original, this shows a
unit of four Na
+
ions and four Cl
ions corresponding to the symbol {(Na
+
)
4
(Cl
)
4
}.
The unit cell with this symbol can be regarded as the smallest unit of the NaCl-
structure - just like the C
2
H
5
OH molecule is regarded as the smallest unit of the
substance ethanol. Symbols like {(Na
+
)
1
(Cl
)
1
}, (Na
+
)(Cl
) or NaCl can be
derived from {(Na
+
)
4
(Cl
)
4
} - they are all shortened representations of the sodium
chloride structure.
Mental models in chemistry: Due to improving knowledge, scientific models
change continuously. Therefore it is not possible to speak of the current atomic
model or the current model of chemical bonds.
Quantum mechanical atomic model
. The structure of the electron shells of atoms
or ions is described by the principal quantum number
n
(“K shell” or “L shell”), the
angular momentum quantum number
l
(s, p, d and f sub-shell), the magnetic
quantum number
m
and the spin quantum number
s
. A maximum of two electrons
with different spins can form an electron cloud or an orbital (Pauli principle).
Starting from the wave-particle duality, wave functions have been developed,
which give information on energy levels and electronic orbitals (Schr
odinger
equation). The calculation of wave functions leads to the description of atoms by
atomic orbitals and molecules by molecular orbitals. In this sense, covalent bonds in
molecules can be described mathematically.
Historical atomic models
. For educational reasons historical models are often
used in chemistry education. For example:
€
- Mass model (Dalton, 1808)
- Mass-charge model (Thomson, 1897)
- Nucleus-shell model (Rutherford, 1911)
- Sub-shell model (Bohr, 1913)
- Electron-pair repulsion model (Gillespie, Kimball, 1966)
