In the recent Chem Activities, we have been learning about how Coulombic attractions and repulsions determine properties of atoms like ionization energy and atomic radius. We have made two assumptions not completely justified by the data we had up to this point: electrons are placed in discrete shells at fixed distances from the nucleus, and only a limited, never changing number of electrons can fit in a given shell. How can photoelectron spectra help us justify placing electrons into discrete energy levels (or shells) and determine the number of electrons that go in each energy level?
Photoelectron spectroscopy is a technique that can provide valuable information about the energy levels and distribution of electrons in an atom. It helps us justify the assumptions about discrete energy levels and the number of electrons that can occupy each level.
To understand how photoelectron spectroscopy works, let's break it down:
1. Energy levels and shells: According to the Bohr model of the atom, electrons occupy specific energy levels or shells, each with a different energy value. However, this model is largely based on assumptions. Photoelectron spectroscopy allows us to verify these assumptions by analyzing the energy of electrons that are ejected from atoms.
2. Experimental setup: In photoelectron spectroscopy, atoms are bombarded with high-energy photons, typically from an X-ray or UV source. When a photon collides with an atom, it can transfer sufficient energy to dislodge an electron from the atom. The energy required to eject an electron is known as the ionization energy.
3. Detecting ejected electrons: The ejected electrons are then collected using an electron detector, which measures the kinetic energy and intensity of the electrons. The kinetic energy of an ejected electron can be related to its original energy level (shell) in the atom.
4. Photoelectron spectra: The data obtained from the detection of ejected electrons is displayed as a photoelectron spectrum, which plots the kinetic energy of the electrons against their intensity. Each peak in the spectrum represents the energy required to remove an electron from a specific energy level.
5. Energy level assignment: By analyzing the photoelectron spectrum, we can determine the energy levels (shells) present in an atom. The positions of the peaks in the spectrum correspond to the ionization energies of electrons occupying different energy levels. This information helps us justify the discrete shells assumed in the model.
6. Electron occupancy: The relative intensity of peaks in the spectrum provides insight into the number of electrons occupying each energy level. Higher intensity indicates a greater number of electrons in that level. By carefully analyzing the spectrum, we can determine the electron configurations of atoms and the maximum number of electrons that can occupy each energy level.
In summary, photoelectron spectroscopy allows us to experimentally determine the ionization energies associated with different energy levels in an atom. The resulting photoelectron spectrum helps justify the assumption of discrete energy levels and provides valuable information about the distribution of electrons, validating the model of electron shells and their occupancy in atoms.