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CHP 5 part 3 - 05-Sep-13 Chapter 5 Lecture 3 of 3 Atomic...

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05-Sep-13 1 Chapter 5, Lecture 3 of 3 Atomic Structure and Periodicity • Orbitals in Polyelectronic Atoms • Electron Spin and the Pauli Exclusion Principle • Electron Configurations • Electron Configurations and the Periodic Table Electronic configuration The electronic configuration of an atom gives a detailed arrangement of the electrons that occupy each subshell and orbital of an atom. The notation used to represent electronic configurations is 1 s 2 2 s 2 2 p 6 elipsis the occupied subshells are indicated by their identifying number and letter such as 2 s the number of electrons in the subshell is indicated by the superscript on the letter. Thus, in the example above, the 2 s 2 notation indicates that the 2 s subshell contains two electrons.
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05-Sep-13 2 Electron configurations Definition: The e- configuration describes the way in which the e- in an atom are distributed among the various orbitals. Four rules: 1. The number of electrons in a neutral atom is equal to the atomic number, ____________. 2. Electrons fill orbitals starting with the lowest n and moving upwards, with no more than two electrons per orbital (Aufbau principle) 3. No two electrons can fill one orbital with the same spin (Pauli’s exclusion principle). 4. Hund’s Rule: for ___________ orbitals, e- fill each orbital singly before any orbital gets a 2 nd e-, i.e., e- are distributed in amongst the orbitals of a subshell in such a way as to yield the maximum number of unpaired e-. Electron spin and the Pauli Exclusion Principle So far we have encountered three quantum numbers: n , l , and m l . However, as scientists applied the quantum-mechanic model to experimental data they discovered it was not quite right. There was something missing. Consider the Stern-Gerlach experiment: The experiment involves sending a beam of particles through a magnetic field and observing their deflection. The results show that particles possess an angular momentum analogous to a spinning object, but that takes only one of two values.
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05-Sep-13 3 The Pauli exclusion principle Electrons behave as if they spin on an axis. According to the Pauli exclusion principle, only electrons spinning in opposite directions (indicated by ↑ and ↓) can occupy the same orbital within a subshell. A fourth quantum number, the ________________________ , m s , was established and assigned possible values of +1/2 and -1/2. The Pauli Exclusion Principle: No two electrons in an atom can have the same set of four quantum numbers n , l , m l , and m s . Consequences of the Pauli Exclusion Principle: For a given orbital the values of n , l , m l are fixed. Thus, if we put more than one e- in a single orbital they must have different m s values.
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