08 - Chemistry 1000 Lecture 8 Periodic trends Marc R...

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Chemistry 1000 Lecture 8: Periodic trends Marc R. Roussel
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A qualitatively correct results of the Bohr theory I Some results of the Bohr theory are qualitatively correct, even for multi-electron 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.
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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.
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Effective nuclear charge n in n in Nucleus of atomic number Z inner shell containing electrons Effective nuclear Z eff = Z charge here is -
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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
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Atomic size Orbitals don’t have a sharp cut-off. They are “spongy”. So how can we define an atomic size?
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