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BLBCJM-CH7-07 - Periodic Properties of the Elements Chapter...

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Periodic Properties of the Elements Chapter 7
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8.2 Classification of the Elements
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In 2002, there were 115 elements known. The majority of the elements were discovered between 1735 and 1843. How do we organize 115 different elements in a meaningful way that will allow us to make predictions about undiscovered elements? Development of the Development of the Periodic Table Periodic Table
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Arrange elements to reflect the trends in chemical and physical properties. First attempt (Mendeleev and Meyer) arranged the elements in order of increasing atomic weight. Certain elements were missing from this scheme. Example: In 1871, Mendeleev noted that As properly belonged underneath P and not Si, which left a missing element underneath Si. He predicted a number of properties for this element. In 1886 Ge was discovered. The properties of Ge match Mendeleev’s predictions well.
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When the Elements Were Discovered
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Modern periodic table: arrange elements in order of increasing atomic number. Effective Nuclear Charge Effective nuclear charge is the charge experienced by an electron on a many-electron atom. The effective nuclear charge is not the same as the charge on the nucleus because of the effect of the inner electrons. Development of the Development of the Periodic Table Periodic Table
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Electrons are attracted to the nucleus, but repelled by the electrons that screen it from the nuclear charge. 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 eff ) decreases. As the distance from the nucleus increases, S increases and Z eff decreases. Effective Nuclear Charge 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 2 , Ne: 1 s 2 2 s 2 2 p 6 , and Ar: 1 s 2 2 s 2 2 p 6 3 s 2 3 p 6 . The radial electron density is the probability of finding an electron at a given distance. Effective Nuclear Charge Effective Nuclear Charge
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Consider a simple diatomic molecule. The distance between the two nuclei is called the bond distance. If the two atoms which make up the molecule are the same, then half the bond distance is called the covalent radius of the atom. Sizes of Atoms and Ions Sizes of Atoms and Ions
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As the principal quantum number increases, the size of the orbital increases. Consider the s orbitals. s orbitals are spherical and increase in size as n increases . The spherical symmetry of the orbitals can be seen in the contour plots. Contour plots are connecting points of equal electron density. Sizes of Atoms and Ions Sizes of Atoms and Ions
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Periodic Trends in Atomic Radii As a consequence of the ordering in the periodic table, properties of elements vary periodically.
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