Chem121-Chapter-7-SH

Chem121-Chapter-7-SH - Chapter 7 Periodic Properties of the...

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Chapter 7 Periodic Properties of the Elements © R. Spinney 2009 Introduction Quantum Mechanics describes the electronic structure of atoms which correlates with the arrangement of 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! 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 e - in different subshells. In addition we have subsets of e - ; 1) core e - : 2) valence shell e - :
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Effective Nuclear Charge (cont’d) Primary interaction of e - & nucleus is due to charge: Coulomb’s Law: F = (kQ e Q n )/r 2 However, the valence shell e - do not experience the full nuclear charge, they are shielded by the core e - 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. Valence shell e - ? Do not screen each other as effectively. Why? Effective Nuclear Charge (cont’d) They are the same distance from the nucleus! The “p” e - do not screen “s” e - The “s” e - do screen the “p” e - somewhat due to the small probability area near the nucleus - penetration
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Effective Nuclear Charge (cont’d) Therefore the effective nuclear charge is more accurately given by: Z eff = Z – S where S is a “screening constant” S depends on specific orbitals Effective Nuclear Charge (cont’d) General Trends: - down a column: Z eff increases slightly as the outer valence shell e - can penetrate better - across a row: Z eff increases by ~1 as each atom has added 1 proton to the nucleus and 1 e - to the valence shell (which does not screen) Effective Nuclear Charge (cont’d) i.e. L iB e BCNOFN e Na K Rb
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Atomic Radii We think of atoms as hard spheres like a marble, which we know from Quantum Mechanics is not true. Question: how do we define the size of an atom? 1) Ionic radius: determined from the crystal structure of ionic compounds.
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Chem121-Chapter-7-SH - Chapter 7 Periodic Properties of the...

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