Visualising chemical structure with a simple inexpensive model By Mr. H. O. Gupta

ABSTRACT

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 unit cell.

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 convenience.

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.