When the elements are displayed on a periodic table, the period number shows in which row the element falls. The period number runs from 1 for hydrogen and helium through to 7 for francium, radium, the actinides and the transuranic elements. The period number corresponds with the quantum number of the highest s-orbital the element possesses, from 1s to 7s.
This graph serves no useful purpose, as the information it displays is depicted in far better fashion by the periodic table itself. But it is extremely useful for plotting the properties of the elements on scattergrams with the period number on the x-axis.
