lecture16sf - The Periodic Table Why is it called the...

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The Periodic Table Why is it called the periodic table? What does the word periodic mean? Typically, periodic refers to repetitive events within a time period: monthly, daily, yearly. A periodical is published at regular intervals. Physicists calculate the period of a pendulum--the length of time to go through one cycle. In the chemical periodic table, there is no time period, but rather a number of elements. When placed in order of their atomic numbers, we return to an element that is in many respects similar, but not identical to a previous element. The similar elements are placed in columns, known as groups or families. The rows of the table are called periods. The number of elements in a period (row) varies. There are only two elements in the first period, H and He. The second and third periods have 8 elements each, filling the s and p blocks. The fourth and fifth period have 18 elements each, each period having an additional 10 elements filling the d block (transition elements). The 6th and 7th periods have 32 elements each, each of these periods having an additional 14 elements filling the f block (lanthanides and actinides), with this block of elements listed separately on the bottom of the periodic table. How did Mendeleev’s periodic table differ from the modern periodic table? The elements in Mendeleev’s table were listed in order of atomic weights rather than atomic numbers. Atomic numbers were unknown in 1871. None of the internal structure of the atom was known in 1871. Representative elements and transition metals were lumped together in Mendeleev’s table. Noble gases are completely absent--they were not known in 1871. The first noble gas discovered in 1898 was argon. Gaps were left in Mendeleev’s chart for elements as yet undiscovered in 1871. Mendeleev made predictions for the properties of Sc, Ga, Ge, which turned out to be remarkably accurate once these elements were discovered. Mendeleev discovered the periodic table pattern on the basis of chemical and physical properties of the elements. Combining ratios with hydrogen and oxygen were included in the groups (columns) of his initial tables, showing how elements in the same group combined with oxygen and hydrogen in the same ratio. Today, we have much more knowledge of the underlying basis of the periodic table. Every element in a column (group or family) has the same valence electron configuration.
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Ionization Energy Ionization energy is defined as the energy required to remove an electron from a gaseous atom. X(g) + energy X + (g) + e - Ionization energy data has been measured for all atoms, and this data provides valuable information on atomic properties and the periodic table. Here is a list of ionization energies for the first 20 atoms. IE () H 1312 He 2372 Li 520 Be 899 B 801 C 1086 N 1402 O 1314 F 1681 Ne 2081 Na 496 Mg 738 Al 578 Si 786 P 1012 S 999 Cl 1251 Ar 1521 K 419 Ca 589 A plot of IE vs atomic number yields obvious periodic behavior. For every row of the periodic table, the ionization energy increases from left to right (with some exceptions) reaching a maximum on the noble gases.
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