Mendeleev's first periodic table was published in 1869. He presented the underlying periodic law to the Russian Chemical Society on 6 March 1869, and a printed version of the table appeared in the journal of the Russian Chemical Society later that year.
The table arranged elements largely by increasing atomic weight while grouping substances with similar chemical properties. Its most daring feature was the presence of gaps. Mendeleev argued that these spaces represented elements not yet discovered, and he predicted several of their properties rather than forcing known elements into unsuitable positions.
The 1869 table was not identical to the modern periodic table. It was based on atomic weights, whereas the modern table is ordered by atomic number, a development made possible after the structure of the atom became better understood. Mendeleev later published an expanded and more detailed version in 1871, which is why 1871 can sometimes be confused with the date of the original publication.