1.7 Periodic Trends

Photoelectron Spectroscopy (PES)

Photoelectron Spectroscopy (PES) is a powerful analytical technique used to explore the energies of electrons in atoms, ions, and molecules. PES allows scientists to gain insights into the electronic configuration of elements, shedding light on the different energy levels where electrons reside. It is essentially based on the energy released by electrons emitted from an atom when it interacts with photons (light).

How Does PES Work?

In PES, a sample is bombarded with photons (light particles) of a known frequency. When these photons hit the atoms, they impart energy to the electrons. If the energy provided by the photon is sufficient, it causes the electrons to be ejected from the atom. This process is very similar to the photoelectric effect.

The ejected electrons leave with a certain kinetic energy, which can be measured. By analyzing the energy difference between the photon and the kinetic energy of the emitted electron, the binding energy of each electron can be determined. The binding energy tells us how strongly the electron was held by the atom and helps us understand the atom’s electronic structure.

The photoelectron spectrum that results from this process can tell us:

• The different energy levels of the electrons within the atom.

• The electron configuration of the atom, based on the number of electrons in each orbital.

Let’s break down how to interpret these spectra.

Interpreting a Photoelectron Spectrum

Below, we’ll explore the example of a photoelectron spectrum for carbon. This is similar to what you might encounter in an AP Chemistry exam.

Understanding the Axes

The first step to interpreting a PES graph is to understand its axes.

• X-axis (Binding Energy): The x-axis represents the binding energy, which tells us how much energy is required to remove an electron from an atom. Binding energy is essentially a measure of how tightly an electron is held by the nucleus. The greater the binding energy, the closer the electron is to the nucleus.

  • In a PES, binding energy increases from right to left, which means the electrons that are closest to the nucleus appear on the left.

• Y-axis (Number of Electrons): The y-axis indicates the relative number of electrons in a given energy level. Each peak represents an energy level or orbital, and its height indicates how many electrons are in that level.

Peaks in a Photoelectron Spectrum

In the example PES spectrum of carbon, let’s analyze what each peak means:

1. First Peak (1s Orbital): The peak closest to the left has the highest binding energy, indicating that these electrons are closest to the nucleus. This is the 1s orbital, and in the case of carbon, it contains 2 electrons.

2. Second Peak (2s Orbital): Moving to the right, we find another peak, representing the 2s orbital. Carbon also has 2 electrons in this orbital.

3. Third Peak (2p Orbital): Finally, the next peak corresponds to the 2p orbital. In carbon, the 2p orbital contains 2 electrons.

Therefore, the electronic configuration of carbon based on its PES is 1s² 2s² 2p². This configuration matches the known configuration of carbon and allows us to understand the number and arrangement of electrons in its orbitals.

Big Ideas in Photoelectron Spectroscopy

When interpreting a PES:

• The position of each peak indicates how much energy is required to remove an electron from that sublevel, which directly relates to the electron’s distance from the nucleus.

• The height of each peak shows the number of electrons in that sublevel.

Example Questions to Practice

Let’s go through some example questions that might appear on an AP Chemistry exam:

1. Refer to the photoelectron spectrum of neon. 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 greater attraction to the nucleus than valence electrons.

   • d. Peaks B and C show the first ionization energies in neon, whereas peak A shows the second ionization energy.

   Answer: The correct answer is c. Core electrons are closer to the nucleus, which means they have a higher binding energy, hence why peak A is positioned to the left.

2. Which peak shows electrons closest to the nucleus?

   • Options: A, B, C, D

   Answer: Peak A represents the electrons closest to the nucleus. Electrons that are closer have higher binding energy and are represented by peaks on the left side of the graph.

AP Practice Question – 2019 Exam Example

This is a sample question adapted from the 2019 AP Chemistry exam:

Part (a) of the question asks you to write the electron configuration and identify the element based on its photoelectron spectrum.

• Start by identifying each peak in the graph, starting from the left. Assign each peak to its respective orbital.

• Write the electron configuration for the element, considering the number of electrons represented by each peak.

If you determine that the binding energy is highest on the left, you start with the 1s orbital. As you move to the right, assign each peak to subsequent orbitals, such as 2s, 2p, etc. The electron configuration you derive from this analysis can be used to identify the element on the periodic table.

Summary

Photoelectron Spectroscopy is an insightful tool for understanding the electron configuration of elements. It allows us to visualize the binding energy of electrons and identify where they are located within an atom. By understanding the basic principles—such as interpreting the axes of a graph and analyzing peaks—you will be well-prepared for any related question on the AP Chemistry exam. The key concepts include the relationship between binding energy and proximity to the nucleus, as well as the number of electrons in a given energy level. Keep practicing, and soon you’ll be comfortable with this important analytical technique!