Dmitri Mendeleev

Dmitri Mendeleev and his first published Periodic Table

Mendeleev's original periodic table, published in 1869.	In the 1860's, Dmitri Ivanovich Mendeleev began working on arranging the elements then known by their atomic weights, to organize them into groups possessing similar properties. In 1863 there were 56 known elements with a new element being discovered at a rate of approximately one per year. As a teacher, he was preparing a textbook for his course when he made his most important discovery. As he attempted to classify the elements according to their chemical properties, he noticed a repetitive pattern. Periodicity of elements had been previously identified by other scientists, de Chancourtois in 1862, and Meyer, but not widely known.
	Where a gap existed in his table, Mendeleev took a professional risk by predicting that a new element would one day be found to fill the gap, and went on to deduce its properties. He was right. Three of those elements were found during his lifetime: gallium, scandium, and germanium. The current popular flat periodic table is generally recognized as descending from his initial efforts, but it is known that he expected a three-dimensional form would be an improvement. In1869, Mendeleev made a formal presentation to the Russian Chemical Society, titled The Dependence between the Properties of the Atomic Weights of the Elements, which described elements according to both atomic weight and valence. Key parts of his presentation stated that: Elements arranged according to atomic weight exhibit periodicity of properties; those with similar chemical properties either have similar atomic weights or have their atomic weights increasing regularly; the magnitude of the atomic weight determines the character of the element; discovery of many yet unknown elements must be expected - for example, two elements, analogous to aluminum and silicon, whose atomic weights would be between 65 and 75; and that certain characteristic properties of elements can be foretold from their atomic weights.
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