<p> Unit 2 / Matter Periodicity and the Periodic Table</p><p>Introduction: Arrange a group of elements by increasing atomic mass:</p><p>A regular pattern emerges in which a member of any particular group is repeated every third element. We will, for now, ignore the reversal of Ar and K. The evidence indicates that there is a repeating pattern in the chemical elements based on their atomic masses. The discovery of this "periodicity" has been extremely valuable to the rapid progress of chemistry.</p><p>History: In 1829, the German chemist J.W. Dobereiner had made some attempt at systematically arranging the elements according to their atomic masses. However, many elements were undiscovered at this time and the atomic mass scale was still a matter of controversy. In 1869, Dmitri Mendeleev, a Russian, and Lothar Meyer, a German, independently discovered the principle of the periodicity of the elements and devised the first periodic tables.</p><p>Periodic Law - the properties of the elements are periodic functions of their atomic masses. </p><p>When he first proposed the periodic table, Mendeleev noted that in order to place the elements in their proper chemical family, it was neccessary to leave certain gaps. Mendeleev stated that where a gap had to be left in the periodic table meant that an undiscovered element fit there, and its properties could be predicted from its position in the chart. For example, in Mendeleev's time, a gap between the elements Si and Sn had to be left. The element arsenic, which was then known and came closest in atomic mass to filling the gap, had properties resembling those of the elements phosphorus and antimony, but showed no relationship to the elements silicon and tin. Mendeleev predicted the missing element to be eka-silicon meaning "like silicon." Let's look at Mendeleev's predictions for the properties of eka-silicon and the experimentally determined data for germanium, the element later discovered and placed in that gap.</p><p>Property Eka-Silicon Germanium Atomic Mass (amu) 72 Density (g/mL) 5.5 Formula of Oxide MO2 Density of Oxide (g/mL) 4.7 Formula of Chloride MCl4 Density of Chloride (g/mL) 1.9 Boiling Pt. of Chloride (*C) <100</p><p>On the basis of the concept of chemical periodicity, we can be confident that there are no undiscovered elements between elements 1 and 109. The present form of the periodic table is still, in general, based on the table of Mendeleev. However, in 1913, Henry Moseley, an English scientist, experimentally determined a more exact basis for the arrangement of elements in the periodic table. Moseley found a numerical ordering to exist among the elements. He assigned each element an atomic number based on his experiments. This new ordering removed the discrepancy of Ar and K and several other pairs of elements.</p><p>Modern Periodic Law - the properties of the elements are periodic functions of the atomic number of the elements.</p><p>The value of the periodic table lies in the way that we can predict many things based on the similarities in properties for groups of elements.</p><p>Implications of the Periodic Table: The discovery of the periodic behavior of the elements created some important questions for scientists. If the elements could be placed in such a regular, ordered arrangement, there must be a basic ordered change in the atoms moving from one element to the next in the table. Furthermore, there must be some common characteristic in the atoms of a family of elements that explains the similarity of their properties. Therefore, there must be some relationship between the periodic behavior of the elements and the structure of their atoms. </p><p>Chemistry I Cary Academy W.G. Rushin 2</p>