ch.7 - 10/14/2009 What!are!the!Goals!(Chapter!7)?

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10/14/2009 1 What are the Goals (Chapter 7)? Learn how the periodic table can help us understand the properties and chemistry of the elements. Understand the concept and implications of effective nuclear charge. Learn about periodic trends of the sizes of atoms and their ions. Learn about ionization energies and their periodic trends trends. Learn about electron affinities and their periodic trends. Begin to learn about descriptive chemistry of the elements and related periodic trends. Screening (Shielding) Effective Nuclear Charge “inner electrons shield outer electrons” an outer electron (in an orbital further from the nucleus) inner electrons e- experiences an effective nuclear charge of Z * << Z Z Screening (Shielding) of electrons: s < p < d < f We say that, within a shell (the same value of n for all electrons), s electrons are screened less than p electrons, p electrons are screened less than d electrons, etc…
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10/14/2009 2 Notes: - core electrons screen valence electrons - electrons within a subshell do not screen each other very well (a 2 p electron does not screen other 2 p electrons 11 Na 1 s 2 s 2 p 3 s 2 2 6 1 valence electron 10 ! core electrons 11+ ! Combined effect = 11 ! 10 = +1 The valence electron in a Na atom “experiences” attraction of the effective nuclear charge of at least +1. Penetration The probability of finding electrons belonging to a given orbital close to the nucleus, at least some of the time , decreases with l We say that s electrons penetrate better to the nucleus compared to p electrons Penetration by electrons: s > p > d > f 3 s
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10/14/2009 3 Z * attraction between electrons and nucleus E electron screening penetration & In summary for many-electron atoms : within a shell (same n ), l E 3 s 3 p 3 d 4 s 4 p 4 d 4 f Recall from Ch. 6: l = 0, 1, 2, 3, … s p d f 1 s 2 s 2 p ases Atomic Radius (atomic size) , R A reases, size increa R A (Na) R A (Al) R A (Na) R A (Rb) < > Z * decreases, size increases n inc R A (S) R A (Cs) R A (I) R A (F) R A (O) R A (N) R A (F) R A (Sb) < < < >
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10/14/2009 4 Atomic Size • The size of an atom increases as n increases going down a column of the increases ( going down a column of the periodic table ). • This point was made as we introduced the Bohr model of the atom and then the quantum mechanical model of the atom (Chapter 6) Atomic radii decrease from left to right in a row - Why? • the inability of electrons in the outer shell (highest ) to fully shield other electrons in the same shell from the charge of the nucleus (which of course increases as you move left to right). e- with lower n shield the outer electrons quite well. e with same n and lowe shield the outer e- with same and lower l shield the outer electrons partially. • e- in the same outer subshell shield each other rather poorly.
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10/14/2009 5 Fig. 7.7 Example • Arrange Se, P, As, S in order of increasing atomic radius.
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This note was uploaded on 05/11/2010 for the course CHEM 1201 taught by Professor Cook during the Spring '08 term at LSU.

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ch.7 - 10/14/2009 What!are!the!Goals!(Chapter!7)?

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