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Unformatted text preview: Chemistry 1000 Lecture 8: Periodic trends Marc R. Roussel A qualitatively correct results of the Bohr theory I Some results of the Bohr theory are qualitatively correct, even for multielectron atoms, but need to be reinterpreted. I Recall, from Bohr theory, r n n 2 / Z . I We call a set of orbitals with the same n a shell . I Orbitals in a shell are similar in size, proportional to n 2 / Z eff where Z eff is an effective nuclear charge. I The electron distribution of a closed shell (one in which all the orbital are filled) is spherical. Effective nuclear charge Theorem: The force on a test charge due to a spherical charge distribution is I zero if the charge is inside the distribution, and I equal to the force due to a single charge equal to the total charge of the distribution placed at the centre of the sphere if the test charge is outside the sphere. What this means to us: An electron in a valence orbital feels a force toward the nucleus that is reduced by the repulsive force of the closed inner shells. On the other hand, the valence orbitals have little effect on the inner shells. Effective nuclear charge n in n in Nucleus of atomic number Z inner shell containing electrons Effective nuclear Z eff = Z charge here is Effective nuclear charge Example The effective nuclear charges experienced by the valence electrons in the second row of the periodic table are as follows: Element Li Be B C N O F Ne Z 3 4 5 6 7 8 9 10 Z eff 1 2 3 4 5 6 7 8 Atomic size Orbitals dont have a sharp cutoff. They are spongy. So how can we define an atomic size?...
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 Fall '06
 Marc
 Electron

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