= Periodic law
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>.<Chemistry 2e by Paul Flowers and others>[Section 6.4, Electronic Structure of Atoms, periodicity and electron configurations]{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.<The Periodic Law of the Chemical Elements by Dmitri Mendeleev>[1889 Faraday lecture, numbered conclusions 6–8 and predictions of missing elements]{ref}
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