Visualising chemical structure with a simple inexpensive model By Mr. H.
This article describes a low-cost crystal-structure model, which
is used to represent atomic positions in different types of unit
cell. It consists of a wooden base with holes drilled on both sides.
These holes can receive dowels onto which balls (commercial or
home-made) can be inserted. The model’s use for modelling different
types of packing in crystals, and the structure of metals,
non-metals, ionic compounds, etc., is explained, with examples. This
model is not only easy to operate and inexpensive, but has
pedagogical advantages too, as the student/ pupil can visualise many
salient features of the crystal structure. A list of activities is
given which can be performed with this model, based on a learning
cycle for conceptual understanding.
Various low-cost ways of modelling chemical structure have been reported
(Gordon, 1970). One method uses 11-hole moulded low-density polyethylene
(LDP) balls with special connectors based on ball-and-stick-type and
tangential-type arrangements (Gupta, 1999). Models from Styrofoam balls are
still cheaper and are also not difficult to make (Sanderson, 1962; Nuffield,
1967). But the difficulties in constructing these models are locating
accurate bond positions on the surface of the balls, joining the balls, and
keeping the structure in place once made, especially giant molecules.
Moreover, this construction method requires a lot of balls in order to
visualise the salient points of different structures. We describe here a
simple low-cost model which avoids the above-mentioned difficulties and
helps students of ages 16 to 18 years to visualise the structure of
different metals, non-metals and their compounds, the shapes of molecules,
different packing arrangements in crystals, and to look at packing
arrangements from different angles, etc.
The model The model consists of a wooden platform, dowels, and balls of
different diameters. On the top side of the platform (A), holes are drilled
at the vertices and centres of squares as shown in Figure 1. On the bottom
side (B), holes are drilled at the vertices and centre of regular hexagons
as shown in Figure 2, with the length of the sides increasing according to
the crystallographic positions of different planes. Note that these figures
show the holes for the positions of atoms in a unit cell, but holes for
extra units can be drilled for showing structures containing more than one
These holes can receive friction-fitting dowels, which are pointed for
easier insertion into the balls. The size suggested for the wooden platform
is 15 cm × 15 cm × 2.5 cm, and the dowel size suggested is 15 cm long with
diameter 2 mm; however, the dimensions of the model can be chosen for
The balls can be (a) polystyrene spheres of different sizes available
from suppliers, (b) homemade spheres of different sizes (these can be made
with the help of different-size moulds which take up granular polystyrene
instead of foamed polystyrene – the moulds are heated in boiling water in a
pressure cooker) or (c) PVC hollow balls of different sizes which are easily
available and very cheap.