AP Chemistry Unit 6 Photoelectron Spectroscopy
Enduring Understandings (this is the new College Board language... bear with me....)
The atoms of each element have unique structures arising from interactions between electrons and nuclei.
- The shell model arises from experimental data.
- The shell model forms a basis for understanding the relative energies of electrons in an atom.
- The model is based on Coulomb's Law and qualitatively predicts ionization energies, which can be measured in the lab.
- Understanding how the shell model is consistent with the experimental data is a learning goal for this content, beyond simple memorization of the patterns of electron configurations.
Course Goals and Essential Questions
- How can we learn about things that are to small to see?
- What relationships can I construct among basic concepts, skills, and understandings?
- How can I best assess my own learning and progress?
- How can I better use technology in my learning?
- How can I become a better digital citizen?
- How can I think more divergently, create, innovate?
- How can I use my experience in chemistry to learn to think and communicate clearly, logically, and critically in preparation for college and a career?
- atoms are composed of protons, newtrons, and electrons
- ions are formed when atoms gain or lose electrons
- the removal of the first electron from an atom involves an energy change
- writing formulas for ions and explaining the charge
- using the Bohr model (shells; circles and dots) to represent electrons in atoms of different elements
Essential Knowledge 1.B.1:
The atom is composed of negatively charged electrons, which can leave the atom, and a positively charged nucleus that is made of protons and neutrons. The attraction of the electrons to the nucleus is the basis of the structure of the atom. Coulomb;s Law is qualitatively useful for understanding the structure of the atom.
- Based on Coulomb's Law, the force between 2 charged particles is proportional to the magnitude of the 2 charges (q and q) and inversely proportional to the square of the distance, r, between them. If the 2 charges are of opposite sign, the force between them is attractive; if they are of the same sign, the force is repulsive.
- The first ionization energy is the minimum energy needed to remove the least tightly held electron from an atom or ion. n general, the ionization energy of any electron in an atom is the minimum energy needed to remove that electron from the atom or ion.
- The relative magnitude of the ionization energy can be estimated through qualitative applecation of Coulomb’s Law. The farther an electron is from the nucleus, the lower its ionization energy. When comparing two species with the same arrangement of electrons, the higher the nuclear charge, the higher the ionization energy of an electron in a given subshell.
- Photoelectron spectroscopy (PES) provides a useful means to engage students in the use of quantum mechanics to interpret spectroscopic data and extract information on atomic structure from such data. In particular, low-resolution PES of atoms provides direct evidence for the shell model. Light consists of photons, each of which has energy E = hν, where h is Planck’s constant and ν is the frequency of the light. In the photoelectric effect, incident light ejects electrons from a material. This requires the photon to have sufficient energy to eject the electron. Photoelectron spectroscopy determines the energy needed to eject electrons from the material. Measurement of these energies provides a method to deduce the shell structure of an atom. The intensity of the photoelectron signal at a given energy is a measure of the number of electrons in that energy level.
- The electronic structure of atoms with multiple electrons can be inferred from evidence provided by PES. For instance, both electrons in He are identical, and they are both roughly the same distance from the nucleus as in H, while there are two shells of electrons in Li, and the outermost electron is further from the nucleus than in H
Learning Objectives: Students will be able to......
- explain the distribution of electrons in an atom or ion based upon data.
- analyze data relating to electron energies for patterns and relationships.
Essential Knowledge 1.B.1:2
The electronic structure of the atom can be described using an electron configuration that reflects the concept
of electrons in quantized energy levels or shells; the energetics of the electrons in the atom can be understood by consideration of Coulomb’s Law.
- Electron configurations provide a method for describing the distribution of electrons in an atom or ion.
- Each electron in an atom has a different ionization energy, which can be qualitatively explained through Coulomb’s Law.
- In multielectron atoms and ions, the electrons can be thought of as being in “shells” and “subshells,” as indicated by the relatively close ionization energies associated with some groups of electrons. Inner electrons are called core electrons, and outer electrons are called valence electrons.
- Core electrons are generally closer to the nucleus than valence electrons, and they are considered to “shield” the valence electrons from the full electrostatic attraction of the nucleus. This phenomenon can be used in conjunction with Coulomb’s Law to explain/rationalize/predict relative ionization energies. Differences in electron-electron repulsion are responsible for the differences in energy between electrons in different orbitals in the same shell.
Learning Objectives: Students will be 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.
- explain the distribution of electrons using Coulomb’s Law to analyze measured energies.
Daily Learning Activities
- Find a partner and complete the Coulombic Attraction POGIL.
- Watch this video by Paul Anderson on Coulombic Attraction, which leads into PES.
- Go to this Google doc and read it.
- Next, go to this Google doc and follow the directions to learn to interpret a PES graph.
- Link to the animation we used in class: PES flash
- Follow the directions.
- PhET simulation Build-an-atom (in HTML5)
- PES Overview
Links and Resources
- Textbook: There is no mention of Photoelectron Spectroscopy that I can find.
- Your reading, above, was taken from Wikipedia (by someone at the College Board.)