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Learning Targets/Content Objectives:
Learning Targets/Content Objectives:
I can determine the electron configuration of an element given its photoelectron spectrum.
I can related the position of a peak in a photoelectron spectrum to the attraction between electrons and the nucleus.
I can explore atomic structure using data from photoelectron spectrum.
I can determine the identity of an element represented by a PES.
What is Photo electron Spectroscopy (PES)?
What is Photo electron Spectroscopy (PES)?
Photoelectron spectroscopy (PES) is a technique that is used to gather information about the electrons in an atom. PES allows scientists to determine the ionization energy of not only valence electrons in the atom, but all electrons. In PES, a gaseous sample of atoms is bombarded with X-ray radiation or Ultraviolet light of known energy. Some of the radiation are absorbed and electrons are emitted. The electrons are collected and their energy is analyzed. During PES, we know the energy of the wavelength of the x-ray absorbed by the electrons. The particular wavelength of x-ray used to remove each electron within the substance gives scientists an idea of the number of electrons in shells and how tightly they are held to the nucleus and thus, the identity of an element. This is known as photoelectron spectroscopy.
Therefore, X-ray radiation used in photoelectron spectroscopy gives scientists information about how tightly the electrons are held by the nucleus by measuring their binding energy and that gives the identity of the element.
Diagram depicting the instrumentation of the X-ray photoelectron spectrometer.
Xray radiation absorbed by electrons make the electrons excited and then they leave the atom
What is a photoelectron spectrum? How does it look like and what information does it show?
What is a photoelectron spectrum? How does it look like and what information does it show?
The spectrum to the right is for Lithium.
The x -axis has units of binding energy or Ionization energy of the electron and is measured usually in electron-volts (eV) or in KJ/mol and it shows how tight the electron is attracted by the nucleus of the atom.
Recall that electrons in higher sublevels have less binding energy, because they are farther away from the nucleus which makes them easier to remove. This means the peak at lower energy (at the right in this spectrum) corresponds with the easiest-to-remove electrons (for lithium, this is the 2s electron). The peak at the left corresponds with the electrons that are hardest to remove (for lithium, these are the two 1s electrons).
The y -axis indicates the number of photons emitted with that energy. This means the height of each peak is proportional to the number of electrons in the corresponding sub-level.
Notice that the peak at 6.26 eV (the 1s electrons) is twice as high as the peak at 0.52 eV (the 2s electron). This means there are twice as many electrons in the 1s sublevel as in the 2s sublevel. The only element that matches this spectrum is lithium (1s2 2s1 ).
Note that the peak at 6.26 eV has higher binding energy than the peak at 0.52eV and that means that peak at 6.26 has electrons that are held more tightly by the nucleus of the atom.
Check your understanding
Check your understanding
What element is represented by the following spectrum? Justify by writing the electron configuration
1s2 2s2 2p6 3s2 3p4
Sulfur
What element is represented by the following spectrum? Justify by writing the electron configuration.
1s2 2s2 2p6 3s2 3p3
Phosphorus
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