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03PeriodicProperties-2

03PeriodicProperties-2 - Chemical Periodicity the Periodic...

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Chemical Periodicity & the Periodic Table Chemical Principles of Marine Systems SIO141/CHEM174 Thursday, October 4, 2007 1
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Can use WebElements to get this http://www.webelements.com 3
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Energy of an electron in an atom is f(z 2 /n 2 ) Thus would expect IE(Li) > IE(H) ? It’s clearly not! IE(Li) = 513.3 kJ mol –1 , whereas IE(H) = 1312 kJ mol –1 Why? Shielding & Ionization Energies 4
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Shielding & nuclear charge Shielding or screening of the nucleus occurs, caused by the inner electrons so that the valence electron only “sees” part of the nuclear charge: an effective nuclear charge ( Z* = z – S ). 5
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Slater’s rules (empirical) 1. Z* = z - S, where z is the atomic number of the element and S is a screening (or shielding) constant. 2. To determine S, electrons are divided into the following groups, each having a different shielding constant: 1s; 2s, 2p; 3s, 3p; 3d; 4s, 4p; 4d; 4f; 5s, 5p; etc. Thus the s and p of a given shell are grouped together, but the d and f are separated. The shells are considered to be arranged from inside out in the order named. 3. The shielding constant S is formed as the sum of the following contributions: a. nothing from any shell outside the one considered; b . an amount 0.35 from each electron in the group considered (except in the 1s group, where 0.30 is used instead); c . if the shell considered is an s , p shell, an amount 0.85 from each electron in the next inner shell, and 1.00 from all electrons still further in; but if the shell is a d or f , an amount 1.00 from every electron inside it.
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