The periodic law states that when elements are arranged according to their atomic number, physical and chemical properties repeat. This periodic law was developed in 1869 by Dmitri Mendeleev and Lothar Meyer.
The table created by Mendeleev left blank spots for new elements to be places as they were discovered. The scientist was able to hypothesize that there would be elements discovered that fit between currently known elements due to the difference in atomic weights from element to element, though there was no space left for noble gases, because they were not known of at the time. The noble gases, like helium, were not discovered until the mid to late 1890s.