Chemistry Reference
In-Depth Information
Fig. 10.3 Johnstone's triangle for chemical education [
3
]
silver crystal like balls in a close-packing of spheres. This model provides a simple
and appropriate image of the silver crystal structure, but also an idea of an infinite
crystal structure and of a giant chemical structure. Since solids play an important
role in elementary lessons, certain structures can be illustrated easily this way. The
silver structure can be described with close-packed Ag atoms on the level of
Dalton's atomic model. The description of metal bonding can be added later with
the help of the differentiated atomic model of nucleus and shell.
Illustrations of finite structures of molecules are also possible on the level of
Dalton's atomic model: different balls represent models for different kinds of
atoms. Molecular structures can be imagined and build on this basis; for example,
a molecular model for an ethanol molecule is built by three kinds of spheres that
represent C atoms, O atoms, and H atoms. Besides these educational reasons,
there are also aspects of educational psychology that suggest the interpretation of
chemical reactions with the help of chemical structures. According to Piaget,
students at the age of 13-16 are in the concrete operational stage: bonding
theories are too abstract and not comprehensible for them, but
illustrations
with packing of spheres or molecular models are possible at
this stage of
development.
From the viewpoint of educational psychology, the arguments in the direction of
chemical bonds have to be set on a higher formal operational stage. If protons,
neutrons, and electrons are being introduced with the differentiated atomic model, it
is extremely difficult to make an arrangement of electrons in shells or energy levels
clear and understandable; electron density distributions, probabilities of finding
electrons, or the wave-particle duality would have to be introduced. Scientifically,
it seems impossible to illustrate electrons as small particles or with the help
of balls for concrete models; today physicists, chemists, or theorists calculate
energies or energy distributions to determine the charge distribution in lattices or
molecules.
