Chem121-Chapter-7_Beekman_SH_Part1

Chem121-Chapter-7_Beekman_SH_Part1 - Chapter 7 Periodic...

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1 Chapter 7 Periodic Properties of the Elements © R. Spinney 2010 Introduction Quantum Mechanics describes the electronic structure of atoms which correlates to the Periodic Table. This connection between electronic structure and chemical & physical properties allows us to make general statements about a number of properties of atom based to their position in the periodic table. A number of elements were discovered based on expected predictions of these “missing” elements.
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2 Effective Nuclear Charge As we have seen, in poly e - atoms, the subshells for any given n have different E. This is due to the different e - - e - repulsions for in different subshells. In addition we have subsets of e - : 1) core e - : chemically inactive, located close to nucleus. 2) valence shell e - : chemically active and located furthest from the nucleus. Effective Nuclear Charge Interaction of e - & nucleus is due to charge: Coulomb’s Law: F = (kQ e Q n )/r 2 i.e. , hydrogen, the single 1s electrons interacts with the single proton in the nucleus. + - 1s
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3 Effective Nuclear Charge (cont’d) Compare H, He and Li: - H: 1s 1 in 1s orbital - He: 1s 2 in 1s orbital: - decreases in size due to stronger e - - nucleus attraction - Li: 1s 2 2s 1 – 1s is smaller but 2s is bigger than H or He 1s + - 1s 1 + - 1s 2 + - 1s 2 2s 1 + - + - + - Effective Nuclear Charge (cont’d) The effect for Li is that the 2 core electrons “cancel” the charge of two protons in the nucleus, thus the “effective nuclear charge” the 2s electron interacts with is only +1. Core electrons shield the valence shell electrons decreasing the nuclear charge. + - 1s 2 2s 1 + - + - + - 1s 2 2s 1 + - + -
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4 Effective Nuclear Charge (cont’d) This same process holds for all other atoms, i.e. Na The 10 core electrons (1s 2 2s 2 2p 6 ) cancel the effects of 10 protons in the nucleus leaving an effective nuclear charge of +1. + - 1s 2 2s 2 2p 6 3s 1 + - + - - + + + + + + + + - - - - - - - + - 1s 2 2s 2 2p 6 3s 1 + - + - - + + + + + + + + - - - - - - - Effective Nuclear Charge (cont’d) This assumes that the core electrons shield the valence shell electrons completely, however this is not true. Why? Valence shell electrons can penetrate closer to the nucleus than core electrons for some types of orbitals.
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5 Effective Nuclear Charge (cont’d) For Li the 2s orbital has a small area closer to the nucleus than the 1s orbital. This allows the 2s electrons to be closer to the nucleus than the 1s electrons for some small portion of time. When this happens the 1s electrons are not shielding the 2s electrons. For Na a similar process occurs for the 3s electrons. Effective Nuclear Charge (cont’d) The effective nuclear charge Z approximated by: Z = # protons - # core e - This is good for core e - which screen valence shell e - most effectively.
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This note was uploaded on 04/01/2012 for the course CHEM 121 taught by Professor Wyzlouzil during the Spring '07 term at Ohio State.

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Chem121-Chapter-7_Beekman_SH_Part1 - Chapter 7 Periodic...

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