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1 Atomic Structure and Properties 2 Compound Structure and Properties 3 Properties of Substances and Mixtures 4 Chemical Reactions 5 Kinetics 6 Thermochemistry 7 Equilibrium 8 Acids and Bases 9 Thermodynamics and Electrochemistry

1 Atomic Structure

1.1 Moles and Molar Mass 1.2 Mass Spectra of Elements 1.3 Elemental Composition of Pure Substances 1.4 Composition of Mixtures 1.5 Atomic Structure and Electron Configuration 1.6 Photoelectron Spectroscopy 1.7 Periodic Trends 1.8 Valence Electrons and Ionic Compounds

Photoelectron Spectroscopy

Learning Objective 1.6.A Explain the relationship between the photoelectron spectrum of an atom or ion and: i. The electron configuration of the species. ii. The interactions between the electrons and the nucleus.

Quick Notes

  • Photoelectron Spectroscopy (PES) measures the energy needed to remove electrons from different subshells of an atom.
  • A PES spectrum shows:
    • x-axis = binding energy (energy needed to remove electron)
    • y-axis = relative number of electrons (peak height)
  • Each peak corresponds to a subshell (e.g. 1s, 2s, 2p).
  • Peak position (along x-axis) shows how tightly electrons are held — closer to nucleus = higher binding energy.
  • Relative peak heights are proportional to the number of electrons in each subshell.
  • PES data supports and reflects an atom’s ground-state electron configuration.

Full Notes

Photoelectron Spectroscopy (PES) is an experimental technique used to investigate the electronic structure of atoms.

It allows us to determine how much energy is needed to remove electrons from different subshells within an atom or ion. This energy is called binding energy.

The data from PES is presented as a photoelectron spectrum.

Interpreting a PES Spectrum

Each peak represents electrons in a specific subshell. The height of the peak is proportional to the number of electrons in that subshell. The position of the peak (how far to the right on the x-axis) shows how strongly the electrons in the subshell are attracted to the nucleus of the atom.

AP Chemistry PES example spectrum with labelled 1s, 2s and 2p peaks; heights show electron counts and positions show binding energy

Relationship to Electron Configuration

PES spectra can be directly linked to the ground-state (lowest possible energy) electron configuration of an atom or ion. Each group of peaks matches the electrons in different subshells.

Example: A PES spectrum for oxygen (1s2 2s2 2p4) would show:

Interactions Between Electrons and the Nucleus

Electrons in inner shells are closer to the positively charged nucleus and are more strongly attracted, which means they require more energy to remove (greater binding energy).

Shielding from inner electrons reduces the attraction felt by outer electrons, lowering their binding energy and making them easier to remove.

These effects are visible in a PES spectrum, where:

Worked Example

Element X has a PES spectrum with the following features:

AP Chemistry unknown PES spectrum showing three peaks at different binding energies inside worked example
  • One large peak at high binding energy
  • One medium peak at intermediate energy
  • One medium peak at low binding energy

Interpretation:

AP Chemistry PES spectrum of fluorine with 1s, 2s and 2p peaks annotated and electron configuration 1s2 2s2 2p5
  • High energy peak (2 electrons) = 1s
  • Intermediate peak (2 electrons) = 2s
  • Low energy peak (5 electrons) = 2p → Electron configuration = 1s2 2s2 2p5 → Element F is fluorine

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Matt’s exam tip

In PES questions, be ready to match spectra to electron configurations or explain differences in binding energy. Focus on how close electrons are to the nucleus and how many occupy each subshell. Also, watch the x-axis as binding energy often decreases to the right, which can trip students up.

Summary