There were 60 known elements in 1860 when Mendeleev published his Periodic Table.
The Periodic Table of Elements
CIENTISTS HAD IDENTIFIED over 60 elements by Mendeleev's time. (Today over 110 elements are known.) In Mendeleev's day the atom was considered the most basic particle of matter. The building blocks of atoms (electrons, protons, and neutrons) were discovered only later. What Mendeleev and chemists of his time could determine, however, was the atomic weight of each element: how heavy its atoms were in comparison to an atom of hydrogen, the lightest element.
"I began to look about and write down the elements with their atomic weights and typical properties, analogous elements and like atomic weights on separate cards, and this soon convinced me that the properties of elements are in periodic dependence upon their atomic weights."
--Mendeleev, Principles of Chemistry, 1905, Vol. II
Russian chemist Dmitri Mendeleev .. although he is often regarded as the father of the Periodic Table, Mendeleev himself called his table, or matrix, the Periodic System. Other people, like Londoner John Newlands, Frenchman Alexandre Béguyer de Chancourtois and German Julius Lothar Meyer made important contributions to the first Periodic Table but the main credit goes to Mendeleev. All of them were helped by the publication in 1860 of more accurate atomic weights, as relative atomic masses were then called. There were two main problems about establishing a pattern for the elements. First only 60 elements had been discovered (we now know of over 100) and second some of the information about the 60 was wrong. It was if Mendeleev was doing a jigsaw with one third of the pieces missing, and other pieces bent!
Mendeleev had written the properties of elements on pieces of card and tradition has it that after organising the cards while playing patience he suddenly realised that by arranging the element cards in order of increasing atomic weight that certain types of element regularly occurred. For example a reactive non-metal was directly followed by a very reactive light metal, then a less reactive light metal. The image of a stamp collectors' miniature sheet shows a stamp commemorating the hundredth anniversary of the Periodic Table superimposed on some of Mendeleev published the Periodic System. Shortly after, his ideas were presented to the Russian Physico-chemical Society. They were read by Professor Menschutkin because Mendeleev was ill. His ideas were then published in the main German chemistry periodical of the time, Zeitschrift f?r Chemie.