Learning Objectives LO 1.7 The student is able to describe the electronic structure of the atom, using PES data, ionization energy data, and/or Coulomb’s Law to construct explanations of how the energies of electrons within shells in atoms vary. LO 1.8 The student is able to explain the distribution of electrons using Coulomb’s Law to analyze measured energies.

PHOTOELECTRON SPECTROSCOPY (PES) How does it work? An atom is subjected to x-rays with specific energy. The x-rays have sufficient energy to expel any of the electrons in atom. When an electron is expelled from the atom, its velocity is measured. This allows for the calculation of the electron’s kinetic energy. The difference between the energy of the x-ray and the kinetic energy of the electron will equal the binding energy for that electron. This is repeated numerous times. The analysis will show peaks that correspond to the energies specific to each orbital in the atom. The relative intensity of the peaks will indicate the number of electrons in each orbital.

Photoelectron Spectra A photoelectron spectrum represents the energy required to remove any electron from a neutral atom. The spectrum is often shown on a logarithmic graph, or on a graph that features changes in scale. The graph shows “Relative number of electrons” on the y-axis and “Energy” on the x-axis. Photoelectron spectroscopy is evidence of our orbital model for electrons in an atom.

Binding Energy (MJ/mol) The only ionization energy shown on a PES is the first ionization energy.