Molecular models, 19th-20th century.

These models show the molecular structure of (left to right) ammonia (NH3), water (H2O), and chloromethane (CH3Cl). Each ball represents an atom of a chemical element, and can be connected to other 'atoms' by means of bonds represented by spokes inserted 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.