University:
Massachusetts Institute of Technology
Course:
3.091 | Introduction to Solid-State Chemistry
Academic year:

2019
Views:

453

Pages:

2
Author:

kaidsgreybeards

3.091: Introduction to Solid State Chemistry, Fall 2018 Recitation 6 1 Aufbau principle Quantum numbers are a handy way to account for the electrons that ﬁll up atomic shells, but they can be tricky to keep track of. Instead, we often utilize shorthand to label the quantum numbers: The Aufbau principle is a trick to keep track of the order each subshell is ﬁlled in: © Source unknown. All rights reserved. This content is excluded from our Creative Commons license. For more information, see https://ocw.mit.edu/help/faq-fair-use. The superscript after each subshell indicates the number of electrons in the subshell: each should be full except for the outermost shell, which is ﬁlled based on the number of valence electrons. Example: Write down both the full electronic conﬁguration and the noble gas notation of Br. Br has 35 valence electrons, so we can ﬁll up all the way to Argon. The full conﬁguration, which mostly follows Aufbau, is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p5 Note here that the 3d10 and 4s2 occur in an unexpected order: there are some exceptions to the Aufbau principle. We don’t expect you to memorize these exceptions for 3.091, but know that they are noted in the periodic table! The noble gas conﬁguration just shows the electrons beyond the last full shell: [Ar]3d10 4s2 4p5 2 Electron ﬁlling and box notation We can note the conﬁguration and orientation of the electrons visually using box notation. It’s usually easiest to use the Aufbau principle (or the periodic table!) to write down the electronic conﬁguration and then translate it to box notation. It’s important to recall Hund’s rule: every orbital in a subshell must be singly occupied before any orbital is doubly occupied. 1 3.091: Introduction to Solid State Chemistry, Fall 2018 Recitation 6 Example: Draw the electronic conﬁguration of Br and C in box notation. We wrote the noble gas conﬁguration above, so we can just give the box notation of the valence electrons here: For carbon, the electronic conﬁguration is 1s2 2s2 2p2 . In box notation, we ﬁll up the 1s and 2s states, and populate the 2p according to Hund’s rule: 3 Photoelectron spectroscopy (PES) Photoelectron spectroscopy (PES) is an experimental method used to determine electronic structure. First, a sample is bombarded with high energy photons to ionize the atoms. Then, the kinetic energy of the electrons that are emitted is measured. The binding energy can be determined using Ebinding = hν − KEe− By plotting the relative counts of electrons emitted with various binding energies, the elemental composition of a sample can be determined using PES! Example: Determine which element corresponds to the following PES spectrum: On this plot, the binding energy increases to the left. The electrons closer to the nucleus should have the highest binding energy, so the left-most peak must correspond to the 1s electrons. The relative height of the peak corresponds to the relative frequency at which the electrons are emitted: since there are two 1s electrons, the height of the left-most peak must correspond to two electrons. The middle peak must correspond to two 2s electrons. Then, since the right-most peak is 1.5 times taller, it must represent 3 electrons in the 2p subshell. The electronic conﬁguration for the element in this PES diagram must be 1s2 2s2 2p3 , which corresponds to nitrogen. 2