The periodic law states that properties of chemical elements recur systematically when the Elements are ordered by atomic number. Similar outer electron configurations help explain recurring chemical behavior. For example, lithium, sodium and potassium belong to the same periodic group and commonly form singly charged positive ions.[ref]
Dmitri Mendeleev originally organized the pattern largely using relative atomic mass and chemical similarities. His 1889 lecture recounts predictions of undiscovered elements from gaps in the arrangement. The modern atomic number formulation supplies a more fundamental ordering than mass alone; the historical predictions show that the scheme could constrain unknown chemistry, rather than merely list known substances.[ref]
The Periodic Law of the Chemical Elements by Dmitri Mendeleev. 1889 Faraday lecture, numbered conclusions 6–8 and predictions of missing elements
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