The periodic table's name comes from the fact that it arranges the elements into repeating sets, otherwise known as "periods." These periods are defined by the covalence of an element, the number of electrons it has in its outermost shell and by other elemental attributes. This arrangement places elements with similar chemical properties close to one another.
Periods are often defined as recurring sets of behavior. Russian chemist and inventor Dmitri Mendeleev was one of the first scientists to realize the attributes that certain groups of chemical elements had in common. He used this observation and formed one of the earliest versions of the periodic table. Mendeleev then sorted the elements according to these patterns to create the periodic table as it is known today.
Mendeleev's table did not include all of the elements that it does now, as many elements had yet to be discovered at that time. However, his table was able to predict that newly discovered elements would fit certain profiles and he designed the table to accommodate this. For this reason, scientists have been able to regularly update the periodic table by adding new elements alongside substances with similar properties instead of replacing the table with another model altogether.