Photoelectron Spectroscopy (College Board AP® Chemistry)
Study Guide
Written by: Martín
Reviewed by: Stewart Hird
Photoelectron Spectroscopy
Photoelectron spectroscopy (PES) is an experimental technique that is used to measure the relative energies of electrons ejected by a source of radiation
Binding energy is the energy required to remove an electron by overcoming the coulombic attraction to the nucleus
A PES spectrum is diagram that shows the relation between the binding energy (x-axis) and the relative number of electrons removed (y-axis)
A PES spectrum is directly related to the electron configuration of an atom
Peaks in the PES spectrum correspond to the energy needed to remove an electron from each shell
Electrons in different subshells experience different effective nuclear charge, affecting the energy
The height of the peaks in a PES spectrum determine the amount of electrons removed from a subshell
Interpreting a PES Spectrum
Photoelectron Spectroscopy (PES) provides insight into the shells and subshells of electrons within an atom or ion
Lower binding energy indicates electrons in subshells closer to the nucleus, which require less energy to remove
Peaks with a greater number of electrons in a subshell will be taller
Examiner Tips and Tricks
Interpreting a PES spectrum is assessed in most of the AP Chemistry Examinations, so make sure to understand its principles and how to predict the electron configuration of an element by analyzing the spectrum
Worked Example
A PES analysis of an unknown element (X) was performed and the spectrum below was obtained. The sample does not contain any impurities. What is the identity of X?
Unknown sample PES Spectrum
Answer:
Step 1: Analyze the number of peaks
Since there are three peaks, this will correspond to the three first subshells of an atom: 1s, 2s and 2p
Step 2: Analyze the position of the peaks
The peak with the highest binding energy will correspond to the 1s subshell since it is the closest to the nucleus, and the coulombic attractions are stronger
The peak with the lowest binding energy will correspond to the 2p subshell since it is the farthest to the nucleus, and the coulombic attractions are weaker
Therefore, the peak in the middle will correspond to the 2s subshell
Step 3: Analyze the height of the peaks
When comparing the height of the three peaks, they are all the same.
Therefore, the same number of electrons have been removed in the 1s, 2s and 2p
Since, subshells 1s and 2s can hold up to 2 electrons, the amount of electrons in the 2p subshell for this atom was 2
Step 4: Predict the electron configuration of the atom
1s2 2s2 2p2
We can connect the electron configuration with the PES Spectrum labeled below
PES Spectrum labeled
Step 5: Identify the element using the periodic table
There are two ways to identify the unknown element:
Counting the electrons and calculating the protons
6 electrons and, since it was a neutral sample, there are 6 protons
OR
Counting the valence electrons and identify the period
There are 4 valence electrons, therefore the element should be in Group 14 of your AP Chemistry Periodic Table
Since the valence electrons are in the second shell, the element should be in the second period of the periodic table
Therefore, element X is carbon (C)
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