Photoelectron Spectroscopy in Advanced Placement Chemistry

Jul 22, 2014 - Photoelectron spectroscopy (PES) is a new addition to the Advanced Placement (AP) Chemistry curriculum. This article explains the ratio...
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Photoelectron Spectroscopy in Advanced Placement Chemistry James Benigna* The Roeper School, Birmingham, Michigan 48009, United States S Supporting Information *

ABSTRACT: Photoelectron spectroscopy (PES) is a new addition to the Advanced Placement (AP) Chemistry curriculum. This article explains the rationale for its inclusion, an overview of how the PES instrument records data, how the data can be analyzed, and how to include PES data in the course. Sample assessment items and analysis are included, as well as resources for teachers to acquire more information about PES and PES data sources. This contribution is part of a special issue on teaching introductory chemistry in the context of the advanced placement (AP) chemistry course redesign.

KEYWORDS: First-Year Undergraduate/General Chemistry, Physical Chemistry, Curriculum, Testing/Assessment, Textbooks/Reference Books, Atomic Structure, Instrumental Methods, Spectroscopy

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more instructional time devoted to core concepts in order to deepen students’ conceptual understanding,6 and improvements to the laboratory program “to situate learning in activities that reflect the kinds of thinking and problem solving in which scientists engage.”7 Scientists rely heavily on analysis of data and instrumental methods of data acquisition, so a conscious effort was made by the AP Chemistry redesign committee to use experimental data throughout the redesigned course. PES provides data that teachers can use to build many concepts that are central to the understanding of atomic theory: electron energy levels and sublevels, effective nuclear charge, electron shielding, electron distributions within sublevels, periodic trends, and electron configurations. Many previous articles in the Journal have presented memory aids and algorithms to help students with using the Aufbau principle for predicting the filling sequence of orbitals in order to generate electron configurations for elements.8−15 The number of articles focused on helping students with this concept points to a large number of instructors experiencing student deficiencies in understanding this curricular topic. Other articles have questioned the merits of teaching a set of rules for electron configurations, when so many exceptions occur and when experimental work in chemistry rarely involves isolated gas phase atoms of pure elemental samples.16−18 In an effort to clarify the concepts of atomic structure, the redesign committee decided to include PES data as a means for

n the redesign of Advanced Placement (AP) Chemistry for the 2013−2014 academic year, Big Idea 1 of the AP Chemistry Course and Exam Description1 references the inclusion of data from photoelectron spectroscopy (PES) in the revised AP Chemistry course (ref 1, p 13): Photoelectron spectroscopy (PES) provides a useful means to engage students in the use of quantum mechanics to interpret spectroscopic data and extract information on atomic structure from such data. In particular, low-resolution PES of atoms provides direct evidence for the shell model. In conferences and on the AP Chemistry Teacher Community site,2 many instructors have raised questions about the inclusion of this data, both in terms of how best to utilize this data in their pedagogy as well as the rationale for including a topic that has not been presented universally by the approved textbooks for the course3 (although many of the textbooks on the list have included discussions of PES in their most recent editions). This article seeks to present the rationale for inclusion of PES data in the course, an overview of how the instrument collects data, as well as strategies for teachers to use the data in their classrooms. This article also includes sample assessment items to provide illustrations of how PES data can be used to evaluate students’ understanding of atomic structure principles.

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WHY INCLUDE PES? In 2002, the National Research Council published a comprehensive examination of the advanced science and mathematics programs in U.S. high schools.4 In their recommendations for the AP program, they included a call for a stronger organizing structure for the course concepts,5 © 2014 American Chemical Society and Division of Chemical Education, Inc.

Special Issue: Advanced Placement (AP) Chemistry Published: July 22, 2014 1299

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instructors to build a mental model of atomic structure with the backing of experimental data. By examining and interpreting spectra from various pure elemental samples, students and teachers can build the concepts of atomic structure with the support of data. Part of the redesign effort placed an emphasis on the science practices, several of which revolve around data collection, data analysis, and supporting claims and predictions with evidence.1 PES data provides a means by which students can support claims and understanding about electronic structure with scientific data, which aligns with Science Practice 6. The redesign committee viewed that PES data provided clearer support for the quantum mechanical model than the data sources that currently appear in most textbooks that are used for AP Chemistry courses: first ionization energy, successive ionization energies, and the Balmer series for hydrogen. PES data provides evidence to support both the shell and subshell model of the atom, as well as to build the concept of atomic orbitals and electron configurations.

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OVERVIEW OF PES Several articles on the technical specifications and uses of photoelectron spectroscopy have already appeared in the Journal,19−22 and entire books are available on the topic as well.23−25 What follows is a cursory overview of the setup and uses of the instrument for the benefit of the reader and to facilitate with the data analysis discussed later.

Figure 1. Schematic of the photoionization process in PES. The length of the KE arrow indicates the difference in photoelectron kinetic energies that arise upon photoionization from orbitals with different binding energies.

standing is not required for the AP course and exam, but students should understand how the instrument can measure binding energy, which is a combination of photoionization and conservation of energy. If the work term is eliminated from eq 1, then students can interpret the process through the following conservation of energy expressions:

Theory and Instrumentation

PES exploits the photoelectric effect that was characterized by Albert Einstein in one of his 1905 papers. When electrons from an atom are exposed to light of sufficient energy, they will be ejected from the electron cloud. Valence electrons are least bound to the nucleus of the atom, so they are ejected with the lowest frequencies (lowest energies) of electromagnetic radiation, often ultraviolet light. In order to characterize all of the electrons in an atom, higher-energy radiation is needed to provide the energy needed to eject core electrons that are more tightly bound to the nucleus, often X-ray radiation. However, any frequency of sufficient energy to remove a first energy level electron can also remove any of the other electrons in the atom. Since the outer shell electrons are less bound to the nucleus, they will eject from the electron cloud with greater kinetic energy, as represented in Figure 1. The ejected electrons (photoelectrons) move from the sample to a kinetic energy analyzer, which is often a concentric hemispheric analyzer (CHA) that uses a difference in voltage across two hemispheric surfaces to curve the path of the photoelectrons to a photomultiplier and into a detector where the photoelectrons can be counted. The voltage required to deflect the photoelectron’s pathway through the CHA can be measured and then used to determine the kinetic energy (KE) of the photoelectron. If the energy of the incident radiation (hν) is recorded and the KE of the photoelectrons determined, then the binding energy (BE) of the electron can be determined from the following equation: hν = BE + KE + φ

hν

A → A+ + e−

(2)

E(A) + hν = E(A+) + E(e−)

(3)

hν = [E(A+) − E(A)] + E(e−)

(4)

hν = BE + KE

(5)

Equation 2 shows the photoionization process, whereas eq 3 shows the conservation of energy during photoionization. The energy difference between the ground state atom and the ionized atom is represented by the binding energy (BE). The energy of the ejected photoelectron, E(e−), will be completely kinetic energy (KE), resulting in eq 5. Since the frequency of light used in a PES experiment will be a constant value, students should be able to reason that the electrons with a greater binding energy will be ejected with lower kinetic energy, and vice versa. This basic understanding is essential to the interpretation of the spectra produced by PES. Data Collection and Analysis

In order for a sample to be analyzed, ultra-high vacuum is often utilized since ejected photoelectrons would be scattered by any gas particles present between the sample and the opening to the CHA. Depending on the instrument, samples can be irradiated with UV or X-ray radiation, and the detector at the end of the CHA tracks the number of photoelectrons for various kinetic energies. X-ray irradiation is most commonly used for solid surface analysis, although a beam of gaseous particles can be irradiated as well. A plot is constructed of signal intensity (which is directly related to the number of photoelectrons) versus binding energy. Figure 2 shows a few

(1)

In modern instruments, the work term, ϕ, can be dealt with by either analyzing a gaseous sample, which eliminates the work function, or by setting the sample to be electrically conductive with the instrument, whereby the work function can be eliminated since it will be the work function of the instrument, which is removed through calibration. This level of under1300

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Complexities in the Data

examples of the spectrum output from PES run on pure elemental samples.

Auger transitions occur frequently in PES, and while study of such transitions is well beyond the scope of the AP Chemistry curriculum, it is worth knowing the effects these transitions have on the spectra produced from PES. Articles discussing Auger transitions have appeared previously in the Journal,21,26 so only a brief summary appears here. When an electron from an inner shell is removed by an incident X-ray, an electron from an outer shell can drop to the inner shell to fill the vacancy. This transition releases a photon that can cause other electrons to become excited, and occasionally, ejected as photoelectrons. Since these photoelectrons can also pass through the CHA, additional peaks are generated in the spectrum. This causes a noisier spectrum, as there are many potential transitions, especially for atoms with many sublevels. For this reason, simulated spectra are often used by textbooks that present and use PES data, and they represent only the dominant peaks in the spectrum that correspond to complete removal of an electron from a sublevel by an incident photon from the irradiation source. Electron spin also influences the binding energy, depending on the alignment of the electron to the nuclear spin. Again, this is well beyond the scope of the AP Chemistry curriculum, but in larger elements, splitting occurs in the sublevels that have multiple orbitals. The splitting arises due to spin−orbital interactions and can be understood in terms of total angular momentum, which is described in reference books and manuals.23,25 To avoid displaying the splitting, simulated spectra are often presented at lower resolution; under these conditions, the splitting is not noticeable unless dealing with elements containing many electrons. To see the effects of splitting, Table 1 presents the binding energy values of the

Figure 2. Simulated photoelectron spectra for lithium, beryllium, and boron.

Examining the spectra in Figure 2, notice that the x-axis is labeled with the units of MJ/mol. The test development committee decided to use this convention for the AP Chemistry course and exam in order to be consistent with SI units. Literature values often report the values of binding energy in electron-volts (eV), and instructors can convert between these units using the following equivalencies: 10.364 eV = 1 MJ/mol or 1 eV = 96485 J/mol. Additionally, the test development committee adopted the convention of displaying the x-axes in descending order, which is frequently used in the literature. Presented this way, students can imagine the nucleus to be at the intersection of the x- and y-axes, and they can assign the sublevel to the peak based on approximate distance from the nucleus moving left to right across the plot. This should make the interpretation of the spectrum easier for the student. Examining the spectra in Figure 2, students can determine that there are two distinct energy levels for electrons in the lithium atom in order to build or confirm a shell model of the electron cloud. With minimal scaffolding, students can likely derive that the first peak corresponds to the first energy level, and the second peak corresponds to the second energy level, since removing an electron from an energy level closer to the nucleus requires more energy, in accordance with Coulomb’s law. By comparing lithium to beryllium, students can determine that peak height corresponds to the number of electrons in each energy level. By comparing beryllium to boron, students can use their prior knowledge that boron has only two principal energy levels to derive that there must be a subdivision within the second energy level. This provides an opportunity to build the concept of sublevels and orbitals from experimental data. In addition to building the subshell model, students can also compare binding energies of the same sublevel in two different atoms. For example, the 1s sublevel experiences greater binding energy for boron than for beryllium and lithium due to the greater nuclear charge. Pairing PES data analysis with first ionization energy and successive ionization energy data can help students connect concepts of electronic structure across multiple data sources, reinforcing their understanding of the differences in electron energies and atomic structure.

Table 1. Binding Energies for 2p1/2 and 2p3/2 Electrons for Various Elements24 binding energy (MJ/mol) element

2p1/2

2p3/2

11Na

3.0 4.8 28.7 33.7 38.9 179.9 213.8 229.0

3.0 4.8 28.4 33.4 38.5 174.1 193.7 208.0

12Mg 19K 20Ca 21Sc 37Rb 38Sr 39Y

2p1/2 and 2p3/2 electrons for several elements. Examining this data shows that there is very little difference in the binding energy of the p electrons for the elements in the first 4 periods of the periodic table, but the splitting becomes more noticeable as the number of electrons increases. Real versus Simulated Spectra

To see an illustration of the differences between real and idealized spectra, a comparison of the real and simulated spectra is presented in Figure 3. Examining the spectra in Figure 3 reveals many significant differences between real and simulated spectra. First, on the real spectrum, the y-axis is presented as signal intensity, which usually has the units of count/s. For a pure elemental sample, the signal intensity is usually proportionate to the relative number of electrons in a sublevel. Examining the real spectrum 1301

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binding energies with electron sublevels and to compare signal intensity with the relative number of electrons in a sublevel. If instructors desire to use real spectra, care should be taken to label peaks for the benefit of students. Students can still compare relative BE values for different elements in a mixture, and they can make claims about the effects of nuclear charge and electron shielding on BE values in real spectra. For example, using the real spectrum in Figure 3, students could analyze why the 1s peak for oxygen is further to the left than the 1s peak for carbon (oxygen has a larger nuclear charge and attracts the 1s electrons more strongly). Student can also compare the relative positioning of the 2s peaks for argon and aluminum and articulate if the reason is the same as for carbon and oxygen.

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Figure 3. Real and simulated photoelectron spectra for aluminum. The real spectrum25 shows one of the Auger transitions for oxygen (O KLL) and a large background signal.

INSTRUCTION WITH PES

Sequence

As with any curriculum topic, instructors use their discretion to embed topics in a context to make the content meaningful to students. In the AP Chemistry Course and Exam Description, PES data is embedded in Big Idea 1, which deals heavily with atomic structure.1 An introduction to PES and analysis of PES data would integrate best wherever the concepts of atomic structure, electron energy levels and sublevels, ionization energy, and the quantum mechanical model are discussed. If the concepts of electron configuration are not yet covered, students can use PES data to build the need for principal energy levels and sublevels based on binding energy. Several texts and instructors have published their approaches that use PES data in this way.28−33 Prior to encountering PES data, students should have experience with the concepts of ionization, ionization energy, Planck’s equation, Coulombic attractions (or Coulomb’s law), and principal energy levels for electrons. Then, students can examine data from PES to derive that even within the same principal energy level, electrons can require distinguishably different energies to remove. From there, the concept of subshells can be built, and students can examine differences in peak height to determine relative numbers of electrons within each sublevel. Students can also establish the maximum number of electrons in each sublevel by examining spectra for several elements on the periodic table and can then connect position on the periodic table with electron structure and the electron configuration for an element. If PES data is introduced earlier in the sequence, students can use PES data to first establish a shell model using the spectra in Figure 2 and other elements from the second period. Once students encounter the spectrum for boron (and the elements that follow), students should be able to determine that even in the second energy level, not all electrons require the same energy to remove. If students proceed to the data from elements in the third period, then they can derive that the energy between principal energy levels is quite large and that the difference between sublevels within the same energy level is significantly smaller. A few instructors have used an approach similar to this one with success.31−33 With this approach, PES data can be used as a bridge between the Bohr model of the atom and the quantum mechanical model. Additionally, students can compare the photoelectron spectra of different elements to conclude the effects of greater nuclear charge and greater electron shielding on the binding energy for electrons in various sublevels.

for aluminum, the 2s peak appears larger than the 2p, which seems inconsistent with the expected electron configuration of 1s22s22p63s23p1. The real spectrum shows a large background signal, and if this is removed from the spectrum, the area under each peak becomes more proportionate. Also absent from the real spectrum is the valence shell of aluminum, which is a factor when using solid samples and X-ray photoionization. Since the valence electrons are minimally bound to the nucleus, they leave the atom with extraordinarily high kinetic energy, and they are likely to crash into the walls of the CHA and not reach the detector. Newer instruments can combine UV and X-ray light sources allowing PES studies on valence electrons. Another reason for the valence electron peaks not being visible in a solid sample is that they are involved in bonding, so they do not exhibit the same kinetic energy upon photoionization, as the bonding affects the binding energy, particularly in an electrically conductive sample. To resolve valence electrons, PES can be run on atomized gas-phase samples as long as the frequency of the irradiating wave is tuned slightly above the binding energies of the valence electrons (X-rays are often too energetic for this task). For simulated spectra, teachers can use the first ionization energy of any element to add the final peak into a spectrum if data is not provided for the highest energy sublevel, as the binding energy of a valence electron and the first ionization energy of a singular gaseous atom are the same. Another difference between the spectra is the narrow range of binding energies (BE) on the real spectrum. The BE range is very large for elements with many electron shells, so experiments usually focus on parts of the spectrum that are of interest for the study. All parts of the BE range can be measured experimentally, and the Supporting Information for this article includes BE values for electrons in each sublevel for the first 80 elements. An additional difference in the real spectrum for aluminum is the presence of contaminants. The oxide layer on aluminum gives rise to a visible 1s peak for oxygen. The 2s peak for argon is also present since an ion gun containing argon was used to minimize the oxide layer on the aluminum sample in this experiment, so traces of remaining argon atoms are visible. Many other factors contribute to the complexity of real spectra, and several books in the references cover these topics in great detail.23−25,27 Simulated spectra are easier for students to use and compare, and simulated spectra can be used by students to compare 1302

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The instructional options discussed above correlate with the concepts in Big Idea 1 and several of the science practices from the AP Chemistry Curriculum Framework:1 • (1.5) The student can re-express key elements of natural phenomena across multiple representations in the domain • (3.2) The student can refine scientific questions. • (5.1) The student can analyze data to identify patterns or relationships. • (6.2) The student can construct explanations of phenomena based on evidence produced through scientific practices. • (6.3) The student can articulate the reasons that scientific explanations and theories are refined or replaced. • (6.4) The student can evaluate alternative scientific explanations. Resources

In addition to the resources highlighted above, there are several instructional resources that have been posted in the AP Chemistry Teacher Community.2 Additionally, the College Board has released a self-paced webcast for AP Chemistry teachers that provides background on PES, animations of the photoionization process and generation of spectra, methods for including PES data in instruction, ways to tie PES to the Science Practices and Learning Objectives in the AP Chemistry Curriculum Framework, and sample assessment items.34 A few textbooks appropriate for AP Chemistry have historically included discussions of PES as it pertains to confirming the current atomic model.29,35,36 Many of the new editions of textbooks published in the last year now include discussions and assessment items using PES data. There are also online sources for PES spectra.27,37 If using a search engine for spectra, the following search strings may prove helpful: • XPS • X-ray photoelectron spectroscopy • UVPS • ESCA spectroscopy • ESCA spectra • Photoelectron spectrum • Photoelectron spectroscopy Using search engines will often result in finding real instead of simulated spectra, so Auger transitions and other background peaks will be present. If using real spectra with students, consider highlighting and labeling the peaks with the appropriate sublevel for the students using a table of reference binding energy values.23

the nucleus. The 2s electrons are indicated for Li at the peak around 0.5 MJ/mol and for Na at the peak around 7 MJ/mol. Students then need to reason why the 2s electrons would be more strongly bound to the nucleus in Na than in Li. Since Na has 11 protons exerting an attractive force on the 2s electrons (compared to Li having only 3 protons), more energy is required to remove these electrons from Na. If this question were given as a free response question, students with a more limited understanding of atomic structure might answer that “the 2s electron is a valence electron for Li.” While this is reasoning is not inherently incorrect, it does not fully explain why the 2s electrons in Na are so much more bound to the nucleus than in Li. A more robust explanation would explain the forces of attraction that pull the electrons to the Na nucleus. A follow-up question could be asked using this same data set: students could be asked to explain why the peak heights are different for the 2s sublevel in each of the plots. To answer this question correctly, students would also have to identify which peaks correspond to the 2s sublevel and then articulate that Li has only one electron in the 2s sublevel, whereas Na has a completely filled 2s sublevel since the 2s sublevel is part of a filled inner shell. This is in contrast to Li, where the 2s electron is in the valence shell, so it can be partially filled. Note: The x-axis in these plots is presented with the break to allow the 1s peak for Na to be easily included while still providing enough detail to differentiate the 2s, 2p, and 3s peaks. To answer Question 2, students would have to first consider the difference in nuclear charge between Al and Mg. A correct response would have the peaks shifted to the right of those in the Al spectrum since Mg has a smaller nuclear charge; thus electrons require less energy to be removed. Also, fewer peaks

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SAMPLE ASSESSMENT ITEMS AND ANALYSIS The following questions are provided as examples of assessment items that teachers can use to gauge students’ understanding of PES data analysis and electronic structure, either as an in-class assessment to check for understanding, or as an item on a unit exam to check for mastery. After each question, a brief discussion follows to explain the types of reasoning that students should be using to support their answers. In order to answer Question 1 correctly, students will have to read the spectra to identify the relevant sublevels. Since the xaxis is arranged in descending order, the left-hand peaks represent the inner orbitals that are most strongly attracted to 1303

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into uncovering aspects of the quantum mechanical model. Using experimental data allows students to work from concrete, measured values to build a model of the atom that is consistent with empirical observations. Finally, with a brief introduction to the operations of the PES instrument, students can answer one of the essential questions in science: “how do we know?” In order to train students to think like scientists, they should be expected to make conclusions based on data, to build models that allow them to analyze new situations, and to modify existing models when new, contradictory data are encountered. These goals are achieved by transitioning students from the Bohr model to aspects of the quantum mechanical model using PES data, which is why PES is now included in the AP Chemistry curriculum.

would need to appear in the spectrum for Mg, as the 3p sublevel would be empty. Finally, the 3s peak would need to be the same height since Mg and Al both have two electrons in this sublevel. There are many ways to vary this question in order to assess students’ ability to relate differences in atomic structure to the photoelectron spectrum. One variation of this question would be to ask students to sketch an approximate spectrum for silicon. In this case, the peaks would need to be shifted left to account for the greater nuclear charge, and the peak furthest to the right would need to be twice as high to reflect a second electron in the 3p sublevel. Another variation is to pick an element immediately above or below aluminum, and students would need to show a very significant shift due to the large change in nuclear charge as well as the appropriate number of peaks to reflect the number of electron shells and subshells. In all of these questions, the focus should not be on the exact values for binding energy; the important criteria for evaluation are relative positioning and relative peak height as a reflection of the differences in atomic structure. Also, either of the spectra above can be used by students to determine the maximum electron occupancy of various sublevels in the atom. Note: The x-axis in the second version of the plot is presented with a logarithmic scale, which is another way to decrease the spacing between the 1s and 2s peak while still distinguishing the outer electron sublevels. To answer the parts of Question 3, students would have to analyze the spectra to determine the difference between the principal energy levels and the sublevels within them. The first four elements’ spectra confirm the quantized energy levels within the Bohr model. Students should be able to articulate that boron requires a refinement of the model, since the Bohr model would predict that the three electrons in the second energy level would be at the same energy. The appearance of the additional peak in boron requires a refinement of the model to include a sublevel that is slightly higher in energy but still within the second energy level.

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ASSOCIATED CONTENT

S Supporting Information *

A Word document containing the sample assessment items in this article (“Sample Assessment Items”). Two Excel spreadsheets: one containing the reference peaks for the first 80 elements (“PES peaks compiled”), and one that can generate simulated photoelectron spectra with both a linear and logarithmic x-axis for the first 21 elements (“PES Graph Generator v3”) individually or in pairs for comparison. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The author acknowledges Richard Schwenz, University of Northern Colorado, and Andrew Blechman, Wayne State University and The Roeper School, for helpful conversations and clarifications. The author acknowledges Thomas Silak, Northville High School, for first assembling the PES Graph Generator spreadsheet that was modified by the author for the Supporting Information to this article. The author also acknowledges Annis Hapkiewicz, Okemos High School (retired), for valuable insight on the curriculum redesign.

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CONCLUDING REMARKS Photoelectron spectroscopy provides a data source that can transition students from the Bohr model of the atom that is commonly presented in first-year high school chemistry courses 1304

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(26) Adler, I.; Yin, L.; Tsang, T.; Coyle, G. The smart electron. J. Chem. Educ. 1984, 61 (9), 757−760. (27) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Mullenberg, G. E., Eds.; Handbook of X-Ray Photoelectron Spectroscopy; Perkin-Elmer Corporation: Eden Prairie, MN, 1979. Available online at http://www.kepu.dicp.ac.cn/photo/07sl02/HANDBOOK_OF_XRAY_PHOTOELECTRON_ SPECTROSCOPY%20%20%E5%A5%BD%EF%BC%81.pdf (accessed Jun 2014). (28) Gillespie, R. J.; Spencer, J. N.; Moog, R. S. Demystifying introductory chemistry. Part 1. Electron configurations from Experiment. J. Chem. Educ. 1996, 73 (7), 617−622. (29) Spencer, J. N.; Bodner, G. M.; Rickard, L. H. Chemistry: Structure and Dynamics, 5th ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 2010. (30) Moog, R. S.; Farrell, J. H. Chemistry: A Guided Inquiry, 5th ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 2013; pp 8−51. (31) Gelder, J.; Shells Activity. http://genchem1.chem.okstate.edu/ CCLIEMD09/Shells%20Activity.pdf (accessed Jun 2014). (32) Gelder, J.; Subshells Activity. http://genchem1.chem.okstate. edu/CCLIEMD09/Subshell%20Activity%20050912.pdf (accessed Jun 2014). (33) Bergman, J. M.; Boesdorfer, S. B.; Carver, J. S.; Mumba, F.; Hunter, W. J. F. Teaching atomic theory using photoelectron spectroscopy data. Chem. Educator 2013, 18, 314−318. (34) This webcast is entitled “Exploring Atomic Structure Using Photoelectron Spectroscopy (PES) Data” and can be accessed under the “Other Core Resources” section at the following webpage: AP Central − AP Chemistry Course Home Page. http://apcentral. collegeboard.com/apc/public/courses/teachers_corner/2119. html?excmpid=MTG243-PR-22-cd (accessed Jun 2014). (35) Ebbing, D.; Gammon, S. D. General Chemistry, 10th ed.; Cengage Learning: Belmont, CA, 2013; pp 310−311. (36) Oxtoby, D. W.; Gillis, H. P.; Campion, A. Principles of Modern Chemistry, 6th ed.; Thomson Brooks/Cole: Belmont, CA, 2008; pp 194−197. (37) Photoelectron Spectroscopy. http://www.chem.arizona.edu/ chemt/Flash/photoelectron.html (accessed Jun 2014).

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