Development of the Periodic Table Introduction: The periodic table of elements is a table that organizes all known chemical elements in order of atomic number. During the 1600s, vast knowledge was discovered about the properties of elements and their compounds, and by 1869, 63 elements had been discovered. As more and more elements were discovered, scientists began to recognize similarities between their properties and began to devise means of classification. Thus the periodic table of elements was created. The current periodic table contains 117 elements, but more may yet be discovered. The elements of the periodic table are classified into groups, periods and blocks. Groups refer to items with similar properties and are the vertical columns of the table. The periods are the horizontal lines and illustrate the number of valence electrons in each element (how many electrons are in the outer shell of the element). Where an element is located on the table allows the scientist to easily judge its properties, its reactivity, and its similarities to other elements. Periodic Table of Dmitri Mendeleev: Dmitri Mendeleev, a Russian scientist, is said to be the father of the periodic table. In 1947 Mendeleev, while working on a textbook, began to organize the 63 then-known elements into groups that exhibited similar properties. Mendeleev found it difficult to classify some metals and alkali metals, and as he tried to find a way to classify them, he began to notice that the properties and atomic weights of the halogens and alkali metals shared similar patterns. He then began to investigate extensions of these patterns within the other elements. Mendeleev created a card for each of the known elements that is... middle of card... the first version of the atomic theory, which later allowed John Dalton to expand the atomic theory to create a method for discerning weight atomic, was crucial in the development of the table. It was the discovery of the atomic weight of elements and the classification of their properties that allowed Mendeleev to create his periodic table and discern possible new elements. The work of JJ Thompson and James Chadwick was also important in creating the "finalized" version of the periodic table. table. The discovery of the electron and then the proton and neutron not only allowed scientists to begin trying to create a detailed model of the atom, but also allowed Henry Moseley to note the structure of protons among the elements and thus create a more accurate representation of the atom. Mendeleev's table sorting elements by atomic number rather than atomic weight.