The periodic table (here by Pedro Nogueira Photography) is one of the most enduring images of chemistry, adorning textbooks, classroom walls, and laboratories alike. Mendeleev first noticed that if you wrote a list of the elements in increasing size (he used mass, today we use atomic number), there is a repetition of properties. So based on this he wrote these into a two dimensional chart, where reading left to right you get increasingly large elements, and elements in a column has the same properties. It turns out this repetition in properties is based on a repeating pattern of how electrons are arranged in subsequent energy levels. Here different regions of the chart are color coded, with, for instance, the noble gases in orange and the transition metals in blue.