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Lec18_11_drobny - Lecture 18 The Aufbau Principle Reading...

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Lecture 18: The Aufbau Principle • Reading: Zumdahl 12.11-12.13 Recommended Problems:12.63, 12.65, 12.67, 12.69, 12.75 • Outline: – Spin – The Aufbau Principle – Filling up orbitals and the Periodic Table
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Spin Further experiments demonstrated the need for one more quantum number. Specifically, some particles (electrons in particular) demonstrated inherent angular momentum.
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Spin (cont.) The new quantum number is m s (analagous to m l ). For the electron, m s has two values: +1/2 and -1/2 m s = 1/2 m s = -1/2
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Pauli Exclusion Principle Defn : No two electrons may occupy the same quantum state simultaneously . In other words: electrons are very territorial. They don’t like other electrons horning in. In practice, this means that only two electrons may occupy a given orbital, and they must have opposite spin.
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The Aufbau Principal For polyelectronic atoms, a direct solution of the Schrodinger Eq. is not possible. When we construct polyelectronic atoms, we use the hydrogen-atom orbital nomenclature to discuss in which orbitals the electrons reside. This is an approximation (and it is surprising how well it actually works).
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The Aufbau Principal (cont.) When placing electrons into orbitals in the construction of polyelectronic atoms, we use the Aufbau Principle. This principle states that in addition to adding protons and neutrons to the nucleus, one simply adds electrons to the hydrogen-like atomic orbitals Pauli exclusion principle:
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