Molecular modelling kit, 1955.
© Science Museum / Science & Society Picture Library
A set of Catalin molecular models of the space-filling Stuart type together with rubber connecting pegs and special pliers in a partitioned case. An engineering drawing showing dimensions and angles is included. Each ball represents an atom of a chemical element, and can be connected to other 'atoms' by means of bonds represented by inserting the connecting pegs in holes in the balls. The number of holes in a ball represents the number of molecular bonds atoms of different elements are able to form. This is determined by an element's valency, which is dictated by the number of electrons in the outermost shell of an atom of that element.