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11/30/2016 Groups The Structure of the Periodic Table II Another way of grouping the elements is by their properties Groups 1 ~ 12: metals 1 - 2: metals Groups 3 - 12: transition metals Groups Groups Groups 13 ~ 18: nonmetals 17: halogens Group 18: noble gases/inert gases Group Organizing Elements by Properties 7 of the 9 elements that lie along the diagonal ‘staircase’ Metalloids Semimetals Semiconductors 13Al, 84Po are the exceptions they’re considered metals Development of the Periodic Table Only 13 elements were identified by the year 1700 As scientists discovered new elements, a way was needed to classify and organize them 1 11/30/2016 Development of the Periodic Table As the number of known elements increased, scientists began to think about organizing them in some way Between 1829 to 1869, many different classification systems were proposed, each organizing the elements in a different way Development of the Periodic Table In 1869, 2 scientists independently published nearly identical classification schemes: Dmitri Mendeleev (Russia) Lothar Meyer (Germany) Development of the Periodic Table Development of the Periodic Table Both The noticed that properties recurred periodically when the elements were arranged in order of increasing atomic mass How is the table currently organized? concept of the atomic number didn’t exist yet The tables developed by Mendeleev and Meyer were the forerunners to the modern periodic table of the elements 2 11/30/2016 Development of the Periodic Table Development of the Periodic Table Mendeleev is given credit for the modern periodic table for 2 reasons: 1) He insisted that chemically similar elements be listed in the same columns (groups) on the table 2) Development of the Periodic Table Development of the Periodic Table In 1913 an English physicist named Henry Moseley discovered the atomic number This occurred 2 years after Rutherford proposed the nuclear model of the atom He left blank spaces on his table where elements didn’t seem to ‘fit’ in terms of their properties He hypothesized that these elements simply hadn’t been discovered yet He was right! Moseley bombarded different elements with electrons and determined the frequencies of X-rays that were emitted He found that each element produces X-rays of a unique frequency 3 11/30/2016 Development of the Periodic Table He arranged the frequencies from lowest to highest, and assigned each element a number -- based on it’s position -- starting at #1 The ‘atomic number’ that he assigned turned out to be exactly equal to the number of protons in the nucleus of each type of atom! Development of the Periodic Table discovery of the atomic number clarified some inconsistencies in Mendeleev’s periodic table It made sense for iodine (I) to come after tellurium (Te) The 53I 52Te 4