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Space-filling models give a representation of the size and shape of the whole molecule, showing (relatively) how much space each atom occupies. Space-filling models were first designed by H. A. Stuart in 1934.

Chemistry students use space-filling models to help when visualising whether the shape of certain bulky structures will prevent them reacting with other molecules. However, space-filling models make it difficult to see how the atoms bond together and prevents seeing the structure of the whole molecule clearly. Ball and spoke representations are much better for showing this information.

Space-filling models use a measurement known as the van der Waals radius G to give the accurate size of each type of atom, based on the density of electrons around them.

» Compare structures made using ball and spoke models, skeletal models and space-filling models

This set of space-filling models in the Whipple Museum's collection was made by Griffin and George, a company that designed mass-produced models for students learning chemistry. Griffin and George's Courtauld Atomic Models set is extremely well known amongst students of the period. It was developed from the designs of Dr. G. S. Hartley of Courtaulds Ltd. in 1952 and underwent several improvements over the next fourteen years. The company Griffin and George had this name only between 1954-1957, so these sets of models can be closely dated to this time.

The models themselves are made of a rigid plastic, with a colour scheme that is different from the recommendations set out by the Institute of Physics. Instead, the colours of these models were chosen for how good they would look in black and white photographs.(1) The set comes with scale cards by Gallenkamp for estimating the size of the molecules (20mm equivalent to 0.1nm).

Models that represent how atoms pack together can be similar in type to the space-filling kind. Atoms can be packed together in several different arrangements, and models are used to represent these arrangements. In the Museum's collection are models that demonstrate hexagonal close packing and cubic close packing.

These particular models are classified as the space-filling type because they model the atoms as hard spheres that are in contact with one another. Although this is not an entirely realistic way to think about atoms, it provides a clear way of visualising the arrangement.

» Find out more about crystal lattice models.

- Griffin
*et al*, 'Courtauld atomic models',*Journal of Scientfic Instruments*Vol. 32 (1955), 195-195. (Find in text ^)

Ruth Horry

Ruth Horry, 'Space-filling models', *Explore Whipple Collections*, Whipple Museum of the History of Science, University of Cambridge, 2008 [http://www.hps.cam.ac.uk/whipple/explore/models/modellingchemistry/spacefillingmodels/, accessed 25 November 2017]

http://www.hps.cam.ac.uk/whipple/explore/models/modellingchemistry/spacefillingmodels/