This new idea of triads became a popular area of study. Between 1829 and 1858 a number of scientists (Jean Baptiste Dumas, Leopold Gmelin, Ernst Lenssen, Max von Pettenkofer, and J.P. Cooke) found that these types of chemical relationships extended beyond the triad. During this time fluorine was added to the halogen group; oxygen, sulphur, selenium and tellurium were grouped into a family while nitrogen, phosphorus, arsenic, antimony, and bismuth were classified as another. Unfortunately however, research in this area was halted by the fact that accurate and precise values of these elements were not always available.
First Periodic Table:
If a periodic table is regarded as an ordering of the chemical elements demonstrating the periodicity of chemical and physical properties, credit for the first periodic table (published in 1862) probably should be given to a French geologist, A.E.Beguyer de Chancourtois. De Chancourtois put pen to paper and made a list of the elements positioned on a cylinder in terms of increasing atomic weight. When the cylinder was constructed so that 16 mass units could be written on the cylinder per turn, closely related elements were lined up vertically. This led the French Geologist to put forward "the properties of the elements are the properties of numbers." De Chancourtois was first to recognize that elemental properties re-occur every seven elements, and using this chart, he was able to predict the stoichiometry of several metallic oxides. Unfortunately, his chart included some ions and compounds in addition to elements.
Law of Octaves:
John Newlands, an English chemist, wrote a paper in 1863 which classified the 56 established elements into 11 groups based on similar physical properties, noting that many pairs of similar elements existed which differed by some multiple of eight in atomic weight. In 1864, Newlands published his version of the Periodic Table and proposed the “Law of Octaves”. This law stated that any given element will exhibit comparable behavior to the eighth element following it in the table.
German scientist Lothar Meyer and the Russian Dmitri Mendeleev both chemists produced remarkably similar results at the same time working independently of one another. Meyer's 1864 textbook included a rather abbreviated version of a periodic table used to classify the elements. This consisted of about half of the known elements listed in order of their atomic weight and demonstrated periodic valence charges as a function of atomic weight. In 1868, Meyer constructed an extended table which he gave to a colleague for evaluation. Unfortunately for Meyer, Mendeleev's table became available to the Scientific Community via publication in 1869 before Meyer's appeared just a year later in 1870.
This was Mendeleev’s version of the Periodic Table:
The Modern Periodic Table:
The elements within the modern periodic table are arranged from left to right, top to bottom, in order of increasing atomic number. An element’s atomic number is the number of protons in its nucleus. There are 92 naturally occurring elements, ranging from hydrogen, which has atomic number 1, to uranium, whose atomic number is 92. The periodic table also includes artificially created elements, whose atomic numbers are higher than 92. Whereas the ordering of the elements is completely determined by their atomic numbers, the arrangement into vertical columns, called groups, is determined by a number of factors. These factors include chemical properties, physical properties, and the number of electrons thought to exist in the outer shells of the element’s atoms. We refer to the horizontal rows in the periodic table as periods. Periods vary in length. Moving through the table from top to bottom, the successive periods contain 2, 8, 8, 18, 18, 32, and 32 elements. These numbers correspond to the maximum number of electrons that can be accommodated in the largest electron shell in an atom of any element belonging to that period.
This is what the Modern Periodic Table looks like:
