AP® Chemistry: PES Data Webcast FAQ
1. Why is Photoelectron Spectroscopy (PES) in the new curriculum when
textbooks and colleges don’t cover this?
PES provides support for a model of the atom that we tell students is valid. Skepticism
and a reliance on data are important characteristics of thinking like a scientist, and
providing this evidence supports students in cultivating those thinking skills. When
students see experimental evidence that is not fully explained by the Bohr model of the
atom, it gives them an opportunity to use data to identify the limitations of a previous
model. It also invites thinking about a different model of the atom. The process of
analyzing new information and its impact on an existing model gives students a real
experience of scientific reasoning.
While colleges and textbooks vary in their treatment of PES data, the AP Chemistry
curriculum includes it because of the crucial importance of explaining to students how
we know that certain scientific models are valid. Textbooks present orbitals and
electron configurations, but they typically do not explain how we know that these
configurations are valid in a way that students can grasp. Textbooks often refer to the
Schrödinger wave equation, but students do not understand how to work with this
equation to arrive at sublevels and electron configurations. PES data is concrete
information that students can understand, and the data confirm electron configurations
and the sublevel model of the atom. That is why PES is in the curriculum.
2. Why does the redesigned AP Chemistry course eliminate the Bohr
equation from the curriculum and put in PES when there is no chance that
students can collect their own data on PES?
Historically, the Bohr model gave us an important building block in atomic structure: that
electron energies are quantized. However, there are fundamental problems with the
Bohr equation. It only describes one atom accurately – hydrogen. It is inaccurate for the
other elements. It also assumes that electrons travel in perfectly circular orbits around
the nucleus, which we know is not accurate. The Bohr equation can be useful in
discussing the historical experiments that led to our current model of the atom, and it
provides a valuable example to illustrate that scientific understanding is open to revision
with new evidence and instrumentation. Both the hydrogen and helium emission spectra
can be demonstrated in the classroom and students can use the Bohr equation to
investigate which energy level transitions might give rise to the observed frequencies of
light. Teachers can guide their students will discover that this can be done for
hydrogen, but helium’s spectrum cannot be described by the equation. This is a
valuable way to demonstrate LOs 1.12 and 1.13:
•
•
1.12 – The student is able to explain why a given set of data suggests, or does
not suggest, the need to refine the atomic model from a classical shell model
with the quantum mechanical model.
1.13 – Given information about a particular model of the atom, the student is
able to determine if the model is consistent with specified evidence.
1
© 2013 The College Board.
After demonstrating that the Bohr model is insufficient to address the emission spectrum
of helium, PES can be introduced as another data source to give further insight into the
structure of the atom. The curriculum committee decided that it served students better to
focus on concrete data that supports our current model of the atom in the AP Chemistry
course and exam.
3. Will the AP Chemistry redesigned exam use simulated or actual spectra?
Both are possible. Due to Auger transitions, sensitivity of the instrument, surface
contaminants on samples, and other complexities in real spectra, simulated spectra
would most likely be used in cases where students are asked to identify elements that
correspond to a spectrum. If real spectra are used, peaks will be labeled to help
students identify which elements or sublevels are responsible for the signal. This will
allow them to focus on analyzing peak position to answer questions about relative
binding energy, nuclear charge, and energy or position of a sublevel relative to the
nucleus or other sublevels.
4. Why do most chemistry books and online sources that simulate spectra
stop at the 3rd shell?
Through the 3rd period of the periodic table, most peaks are singular for each sublevel:
there are definitive peaks for s and p sublevels. At higher resolution, it is possible to see
the splitting of peaks in some sublevels, which results from quantum mechanical effects.
For the first 3 rows of the periodic table, the splitting in the p sublevel is minimal. When
analyzing elements with more energy levels, this splitting becomes more noticeable and
makes the analysis more complicated for the student. To keep the analysis simple, most
sources focus on the first few rows of the periodic table. If larger elements are used,
then teachers should be careful to point out which peaks correspond to which sublevels.
As long as the spectra are sufficiently “zoomed out,” most splitting patterns would
appear to be a singular peak for many elements. If teachers desire to go further into the
quantum mechanical model, then they certainly can examine PES data that
demonstrate the splitting pattern as evidence to support the quantum mechanical model.
5. Will the units on the x-axis for PES data be eV or MJ/mol?
To minimize confusion, the units of MJ/mol will be used on the AP exam since the units
of J/mol and kJ/mol appear elsewhere in the curriculum, and students should have the
metric prefixes as part of their prior knowledge. For graphical interpretation, the precise
units are not of primary importance. If students or teachers are interested in using eV, it
is possible to convert using a rough proportion of 10 eV to 1 MJ/mol.
2
© 2013The College Board.