Lect_Note_chapter_7

Lect_Note_chapter_7 - Chapter 7 Periodic Properties of the...

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Chapter 7 Chapter 7 Periodic Properties of the Periodic Properties of the Elements Elements CHEMISTRY The Central Science Professor Li Niu
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In 2006, there were 118 elements known. The majority of the elements were discovered between 1735 and 1843. How do we organize 118 different elements in a meaningful way that will allow us to make predictions about undiscovered elements? 7.1 Development of the Periodic Table 7.1 Development of the Periodic Table
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Arrange elements to reflect the trends in chemical and physical properties. In 1869, Dmitri Mendeleev and Lothar Meyer arranged the elements in the order of increasing atomic weight – see the Periodic Table next 7.1 7.1 Development of the Periodic Table Development of the Periodic Table
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Periodic Table and I ts Use Periodic Table and I ts Use Elements are arranged in the order of increasing atomic number, the concept developed by Henry Moseley (1887-1915). He found that the frequencies of X rays emitted differ from different elements
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Example: In 1871, Mendeleev noted that As properly belonged  underneath P but not Si, which left a missing element  underneath Si.  He predicted a number of properties for this  element.  In 1886 Ge was discovered by  Clemens Winkler in  1886 .  The properties of Ge matched Mendeleev’s predictions  well ( eka-silicon ). Periodic Table and I ts Use Periodic Table and I ts Use
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Effective nuclear charge is the charge  experienced by an electron on a many-electron  atom  (an averaged environment created by  attraction – between nucleus and electrons –  and repulsions among electrons) The effective nuclear charge,  Z , is not the  same as the charge on the nucleus because of  the effect of the inner electrons.  These  electrons act as a screening of the nuclear  charge. 7.2 7.2 Effective Nuclear Charge
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7.2 Effective Nuclear Charge 7.2 Effective Nuclear Charge
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The nuclear charge experienced by an electron depends on its distance from the nucleus and the number of core electrons. As the average number of screening electrons ( S ) increases, the effective nuclear charge ( Z ) decreases. 7.2 Effective Nuclear Charge 7.2 Effective Nuclear Charge
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The  n s  orbitals all have the same shape, but  have different sizes and different numbers of  nodes Consider : He: 1 s , Ne: 1 s 2 2 s 2 2 p 6 , and Ar: 1 s 2 2 s 2 2 p 6 7.2 Effective Nuclear Charge 7.2 Effective Nuclear Charge
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Fig. 7.3 However, the 3s electrons have some ability of penetrating the core, so the Z eff is actually 3.3+.
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Z eff = Z – S Therefore, the effective nuclear charge experienced by the outer electrons is determined primarily by the difference between the charge in the nucleus and the charge of the core electrons.
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