The Periodic Law of Dmitri Mendeleev

Following de Chancourtois' and Mendeleev's publishing their element arrangements, Mendeleev pronounced the Periodic Law in 1869, formalizing the rules they both recognized.
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Other scientists had previously identified periodicity of elements, and Alexandre-Emile Béguyer de Chancourtois, seven years earlier, had arranged the elements then known in a helix, in three-dimensional 'barber pole' style.
Vis Tellurique museum exhibit
The word periodic means occurring or repeating at intervals.
So the periodic law states that the chemical properties of the elements occur periodically when the elements are arranged in order - in the mid-1800s - of increasing weight.
For example, in a column (family or group) of elements, the elements will have similar chemical properties because the chemical properties are repeating at intervals along the element line.
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Periodic Law card included with 3D Forever
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Carmen Giunta's translation of Mendeleev's words in Russian are: "The elements, if arranged according to their atomic weights, exhibit an evident stepwise variation of properties", but other language experts translate 'stepwise' as being intended to be 'periodicity', a more appropriate term for Mendeleev to use at that time.
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For over a century and a half, regardless of the many different attempts to improve the relation of the Periodic Law and the tabular format of the most common chemistry worksheet, the Periodic Table, the elements in order aspect of the Law had been still unresolved - until 1980, when the Alexander Arrangement of Elements format finally depicted the Periodic Law in an accurate and complete way, halting criticism of the Periodic Table for failing to do so.

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last update: 3/20/2020