Lecture_6 - 11. The Aufbau Principle and the Periodic...

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1 11. The Aufbau Principle and the Periodic Table: Aufbau or “Building up” principle: just as protons are added one by one to the nucleus to build up the elements, electrons are similarly added to the hydrogen-like orbitals. Lower energy orbitals are filled first, in accordance with the ( n + l ) rule. H: 1s 1 Two electrons with opposite spins can occupy the same orbital, in conformity with the Pauli exclusion principle . He: 1s 2 In Li, the outermost e will occupy the lowest-energy orbital next to the 1s. Li: 1s 2 2s 1 Be: 1s 2 2s 2 B: 1s 2 2s 2 2p 1 ↑↓ In Carbon (C), the mutually repulsive e s will occupy separate 2p orbitals. Hund’s rule : the lowest energy electron configuration for an atom is the one having the maximum number of unpaired (or parallel ) electrons.
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2 C: 1s 2 2s 2 2p 2 N: 1s 2 2s 2 2p 3 etc… ¾ Closed shell notation: Na: 1s 2 2s 2 2p 6 3s 1 [Ne] 3s 1 Mg: [Ne] 3s 2 Al: [Ne] 3s 2 3p 1 Ar: 1s 2 2s 2 2p 6 3s 2 3p 6 [Ar] ¾ Valence electrons
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This note was uploaded on 09/07/2010 for the course FAS chem 201 taught by Professor Sultan during the Summer '07 term at American University of Beirut.

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Lecture_6 - 11. The Aufbau Principle and the Periodic...

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