In a video store, the latest movies are usually placed on the shelves in alphabetical order. Older movies are grouped into categories such as Action or Comedy. The manager has to choose a set of categories and then place each movie in the most appropriate location.

Figure 1  Older movies in video stores are placed into categories such as Drama or Comedy.

Scientists faced a similar challenge when they looked for a logical way to organize the elements. They had to decide what categories to use and where to place each element. An organized table of the elements is one of the most useful tools in chemistry. The placement of elements on the table reveals the link between the atomic structure of elements and their properties.