What is the periodic table?
The periodic table of the elements is a representation of all known elements in an orderly array. The periodic law presented by Dmitri Mendeleev in 1869 stated that if the (known) elements are arranged by atomic weight, then certain trends in chemical properties can be observed. That is to say, when the elements are arranged by atomic weight, then elements whose chemical behavior is similar are seen to line up with each other with some regularity. The number of known chemical elements was considerably smaller in Mendeleev’s time. Nor was there an accurate or reliable model of atoms that could describe observed chemical behaviors. The so-called ‘noble gases’ (helium, neon, xenon, etc.) and many other elements were unknown. As more elements were identified, however, Mendeleev’s crude periodic table was expanded and refined. The development of modern atomic orbital theory made it possible to identify and relate chemical behavior based on electronic structures.
