Ball and spoke models

Ball and spoke models are a common way of representing molecular structures. Each atom is represented by a coloured ball that is joined to other atoms using spokes to represent the bonds between them. This type of model emphasises the bonding between atoms.

ball and spoke model kit
Image 1 Molecular model kit 'set B' by Molecular Models of Cleveland, Ohio; made mid-20th century. Image © the Whipple Museum (Wh.6061).
skeletal model kit by Orbit
Image 2 Set of skeletal molecular models by Orbit. Image © the Whipple Museum (HC28).

Showing how atoms bond together

Each wooden ball has a number of holes drilled into it. These correspond with the number of bonds that the atom can form with other atoms. The number of bonds that can be made depends on how many electrons an atom has, which varies depending on the element. For example, oxygen atoms can make two bonds, but hydrogen atoms can only make one.

The chemist Wilhelm August Hofmann first used coloured balls to represent the elements around 1865. John Dalton, XR  who in the 19th century made great advances in thinking about the structure of atoms, used ball and spoke models in his lectures. Some of his models and diagrams of atoms are displayed in the Science Museum in London.

Other 'open' models: skeletal models

Skeletal models are similar to the ball and spoke kind; they are both classified as 'open' structures, as opposed to the 'closed' space-filling type. In skeletal models, the atoms are not shown as spheres. Instead the atoms are assumed to be at the intersection of two or more rods, which represent the bonds. The main advantage of skeletal models that is easy to measure angles and dimensions due to their open structure. The Whipple Museum has a set of Orbit skeletal models, shown in Image 2.

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

Ruth Horry

Ruth Horry, 'Ball and spoke models', Explore Whipple Collections, Whipple Museum of the History of Science, University of Cambridge, 2006 []

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