Mendeleev noted patterns in the combining ratios of elements

• The Elements Lithium (Li), Sodium (Na), and Potassium (K) all formed oxides in the ratio of two atoms per oxygen atom: R₂O
• The Elements Beryllium (Be), Magnesium (Mg), and Calcium (Ca) all formed oxides in the ratio of one atom per oxygen atom: RO
• Boron (B) and Aluminum (Al) formed R₂O₃
• Carbon (C) and Silicon (Si) formed RO₂

Recognizing the patterns of combining ratios or "valency", Mendeleev created a table organized by placing elements with similar combining ratios in the same group.   He arranged the elements within a group in order of their atomic mass. 

In 1869, the Russian chemist Mendeleev noted that the repeating patterns of behavior could be arranged in a sequence of elements giving rise to the "Periodic Table" of the elements.

Special thanks to Dr. Paul Karol's "Intro to Modern Chemistry" for providing much of the information on this page.

  
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