Structural and Mental Models - Essentials of Chemical Education - page 165

In connection with the Chernobyl disaster the caesium isotope Cs-137, which
decays with a half-life t 1/2 ¼ 30 years, was discussed in the newspapers. With the

help of this game students will be able to grasp the consequences: it takes almost
100 years until the Cs-137 activity decays to 10% of the starting value. Luckily the
clouds with radioactive particles spread widely over Western Europe so that experts
predicted the dose of Cs-137 in food to be small despite its long half-life.

6.2 Teaching Processes: Models and Their Functions

Students already gained lots of experience in the field of natural phenomena: this
field is concrete for them. Therefore they like the elementary lessons in biology,
chemistry, geography and physics: they stay in the well-known field of direct
illustration and tangible phenomena. As soon as formulae and reaction equations
play a role in chemistry lessons, interest in chemistry dwindles: chemistry becomes
dry and lifeless, becomes hard to understand. One reason is that formulae and
equations belong to abstract mental models (see Chap. 6.1 ).
Therefore it is important for the education process to find out which display
models, for example molecular models, packing of spheres or crystal structures, can
be used, before mental models are being introduced. All bonding models or models
of the structure of single atoms or ions are abstract mental constructs. They have to
stand back until a first understanding of the structure of matter is build up with the
help of illustrative structural models. First the chemical structure, then the chemical
bond!
Some chemistry and physics educators see the use of particles and ball models
for chemical structures with a critical eye. Buck [ 13 ], for example, calls the particle
model a “nonmodel” and opposes the usual illustration with circles or balls and
their arrangement for demonstrating models of the states of matter or aqueous
solutions: “Actually we are not allowed to draw spheres, because we are dealing
with centers of force. Teachers and authors who should know, accept without
problems that atomic and molecular orbitals are asymptotic, can stretch across
the whole universe, that limiting lines are drawn arbitrarily, mostly at 85%. To
summarize it: the visualisation and illustration by particle pictures is a crucial
mistake” [ 13 ].
He also recommends initiating “the jump” to the atoms by showing slides of
increasing or decreasing complexity and a discussion of system characteristics:
“egg ! hen house ! farm ! village ! country ! earth ! universe ! earth !

city ? What about the next slide? There is no next

slide, because such a slide does not exist” [ 14 ].
This discussion might be very interesting, for sure. It can take place in
class before the introduction of smallest particles. But the consequence of the
“nonexistence of the next slide” cannot be to go over to the abstract “centers
of force” or “endlessly widespread nucleus-shell systems” right away. For deve-
lopmental psychological reasons concrete circles, balls, cubes or lego bricks have

! school ! students ! hair !