Molecular models, c 1875.
© Science Museum / Science & Society Picture Library
This molecular modeling set is labelled after August Wilhelm von Hofmann (1818-1892) and signed 'Manuel Gonzalez, Verdiguier, Madrid'. The set comprises five stands and coloured balls with attached spokes, labelled with chemical formulae and bras collars. The carbon atoms are tetrahedral. 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.