The modern periodic table lists the elements in the order of increasing atomic number, which is the number of protons in the nucleus of an atom.
One of the initial attempts to classify the elements was done by Antoine Lavoisier in 1789. He classified them as gases, non-metals, metals and earths based on their properties. In 1829, triads of elements with chemically similar properties such as sodium, lithium and potassium were recognized by Johann Dobereiner. In the following decades many attempts were made to group elements together.
A pivotal classification at came in 1866 when Russian chemist Dmitry Mendeleev listed the elements by atomic weight. He identified a pattern wherein similar elements appeared at regular intervals or periods. Mendeleev first published his periodic table in 1869 followed by a revised version in 1871. It left gaps where a break in the pattern occurred. He correctly predicted that those gaps represented undiscovered elements.
Julius Lothar Meyer was a contemporary of Mendeleev. Meyer also produced several periodic tables between 1864 and 1870. His 1868 table listed the elements in order of atomic weight, with elements with the same valency arranged in vertical lines, remarkably like Mendeleev’s table. Unfortunately, Meyer’s work was not published until 1870 after Mendeleev’s table had been published.
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