Light exists as both a particle and a wave. Einstein observed the photoelectric effect as light in its particle form.
The photoelectric effect demonstrates that electrons are emitted from a metal surface when light strikes it if the frequency is high enough✨.
If the frequency (v), or # of waves per second that pass a point in space, is low, the metal absorbs the light.
Image Courtesy of Google
Photoelectron spectroscopy (PES) is a method to compare the relative energies of atoms, ions, and molecules.
PES uses energy from electrons emitted through the photoelectric effect to provide insight about the electronic configuration of a sample.
When light of a certain frequency shines upon a sample, a limited number of electrons are emitted. The released energy reflects the energy or energy levels within an atom.
In viewing the PES chart of an element, you are also able to distinguish the different orbital levels and determine the electron configuration. Each of the peaks in a PES chart signifies a different orbital level, and the y-axis of the chart gives the number of electrons.
Here is a diagram putting all of these concepts together:
Image Courtesy of Chemdx
Here is a PES of Carbon without the markups. Let's dissect it.
Image Courtesy of Chemmybear
First, let's look at the axes. The x axis is the binding energy, which is synonymous with the ionization energy on this exam. The ionization energy is the amount of energy required to remove a valence electron.
The closer an electron to the nucleus, the greater the ionization energy since the attraction between the negative electron➖ and positive nucleus➕ is the strongest.
This information should be able to tell us which side the nucleus is on in this diagram. It's on the left side, since the binding energy is greatest there (1000>0.1). So let's read the diagram from left to right.
The first orbital, as always, has to be 1s. Since the graph goes up to 2, there are 2 electrons in the 1s orbital.
The next orbital has to be 2s and the PES indicates there are 2 electrons in this orbital.
So far, the electron configuration seems to be 1s^2 2s^2.
There is one more peak though, which corresponds to the 2p orbital. However, this orbital isn't filled to maximum capacity, there are only 2 electrons in it.
The full electron configuration of this element is 1s^2 2s^2 2p^2. If I didn't tell you that this PES was for carbon, you should be able to guess it given the graph.
The position of the peak indicates how much energy is required to remove an electron from that sublevel.
The height of the peak indicates how many electrons occupy that sublevel.
Refer to the photoelectron spectrum of neon shown below to answer the following question. Which of the following statements best accounts for peak A being to the left of peaks B and C?
a. The electron configuration of neon is 1s2 2s2 2p6.
b. Neon has 8 electrons located in its valence shell.
c. Core electrons of an atom experience a much higher effective nuclear charge than valence electrons.
d. Peaks B and C show 1st ionization energies (I.E.) in neon, whereas peak A shows the 2nd I.E. of Neon.
2. Which peak shows electrons closest to the nucleus? A, B, C or D?
The answer to #1 is C, Core electrons of an atom experience a much higher effective nuclear charge than valence electrons. This goes back to the concept that electrons closest to nucleus have a higher ionization energy/binding energy.
The answer to #2 is peak A. This goes back to that same exact concept. If asked about which peak corresponds to the energies of the valence electrons, you should say peak D.
This question is taken from the AP Chemistry Exam from 2019.
In part a, they are asking you to simply write the configuration and identify the element. We just did this a few times! If you feel comfortable with this, try it on your own first before looking at the answer.
Since the binding energy is largest on the left, the peak on the left is the 1s orbital. The electron configuration is:
You could use the noble gas shortcut here as well, but I often leave the electron configuration like this when given a PES.
To identify the element, just pull out your periodic table! You should get Ca.
🎥Watch: AP Chemistry - Photoelectron Spectroscopy
✍️ Free Response Questions
AP Chemistry Free Response Questions
⚛️ Unit 1: Atomic Structure and Properties
1.1Moles and Molar Mass
1.2Mass Spectroscopy of Elements
1.3Elemental Composition of Pure Substances
1.4Composition of Mixtures
1.5Atomic Structure and Electron Configurations
1.6Photoelectron Spectroscopy & Graph Interp.
🤓 Unit 2: Molecular and Ionic Compound Structures and Properties
2.0Unit 2 Overview: Molecular and Ionic Bonding
2.1Types of Chemical Bonds
2.2Intramolecular Force and Potential Energy
2.3Ionic Bonding and Ionic Solids
2.4Metallic Bonding and Alloys
2.5Lewis Dot Diagrams
2.6Resonance and Formal Charge
🌀 Unit 3: Intermolecular Forces and Properties
3.0Unit 3 Overview: Intermolecular Forces and Properties
3.2Properties of Solids
3.3Solids, Liquids, and Gases
3.4The Ideal Gas Law
3.5The Kinetic Molecular Theory of Gases
3.6Deviations from the Ideal Gas Law
3.7Mixtures and Solutions
3.8Representations of Solutions
3.9Separation of Solids/Mixtures
3.10Solubility and Solubility Rules
3.11Spectroscopy and the Electromagnetic Spectrum
3.12Quantum Mechanics and the Photoelectric Effect
🧪 Unit 4: Chemical Reactions
4.0Unit 4 Overview: Chemical Reactions
4.1Recognizing Chemical Reactions
4.2Net Ionic Equations
4.4Physical vs. Chemical Changes
4.5Stoichiometry & Calculations
4.6Titrations - Intro and Calculations
4.8Intro to Acid-Base Neutralization Reactions
👟 Unit 5: Kinetics
5.0Unit 5 Overview: Kinetics
5.1Defining Rate of Reaction
5.2Introduction to Rate Laws
5.3Rate and Concentration Change
5.4Writing Rate Laws
5.5Collision Model of Kinetics
5.6Reaction Energy and Graphs w/ Energy
5.7Reaction Mechanisms and Elementary Steps
5.8Writing Rate Laws Using Mechanisms
🔥 Unit 6: Thermodynamics
6.0 Unit 6 Overview: Thermochemistry and Reaction Thermodynamics
6.1Endothermic Processes vs. Exothermic Processes
6.2Energy Diagrams of Reactions
6.3Kinetic Energy, Heat Transfer, and Thermal Equilibrium
6.4Heat Capacity and Coffee-Cup Calorimetry
6.5Phase Changes and Energy
6.6Introduction to Enthalpy of Reaction
6.7Bond Enthalpy and Bond Dissociation Energy
6.8Enthalpies of Formation
⚖️ Unit 7: Equilibrium
🍊 Unit 8: Acids and Bases
8.0Unit 8 Overview: Acids and Bases
8.1Introduction to Acids and Bases
Unit 9: Applications of Thermodynamics
🤺 AP Chemistry Essentials
🧐 Multiple Choice Questions
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