Evidence for Energy Sub-Shells in an atom – An Exploration of PES Data
Chem. 319
Ms. Hobbie
One important conclusion that can be reached based on first ionization energy experimental data is
that electrons in shells farther from the nucleus require less energy to be removed. An interesting
question we could ask, one that can’t be answered from first ionization energy data, is:
Do all electrons in the same shell require the same amount of energy to remove?
BACKGROUND
This question can be answered by looking at a different kind of experimental data collected from a
method called photoelectron spectroscopy (PES). Photoelectron spectroscopy is based upon a single
photon-in/electron-out process. In this experimental method, high-energy radiation such as x-rays is
used to dislodge all of the electrons from an atom. Specifically, high-energy monochromatic radiation
(i.e. only photons of a fixed energy value) is shined at an atom. The kinetic energy of the electrons that
are subsequently removed from the atom will vary depending on their binding energy, that is,
depending on the amount of work needed to dislodge the electron. Like in the photoelectric effect,
when more work is required to dislodge an electron, less will be available as kinetic energy for the
electron as it leaves the atom.
Since not all electrons have the same binding energy, there will be a distribution of kinetic energies of
the photoelectrons emitted in this process. The kinetic energy distribution of the emitted photoelectrons
(i.e. the number of emitted photoelectrons as a function of their kinetic energy) can be measured and
a photoelectron spectrum can thus be recorded. Analysis of the kinetic energy distribution of the
emitted photoelectrons can help identify the allowable energy states of the electrons in an atom.
INTRODUCTION TO THE DATA WEBSITE
The site you will be working with is: www.chem.arizona.edu/chemt/Flash/photoelectron.html
Once you get to the website, you’ll see a section of the periodic table containing the first 21 elements.
Clicking on an element symbol will bring up pseudo-PES data for that element. NOTE: This data has
been converted – it does not show KE distributions of the photoelectrons but rather calculations have
been performed to provide you with the binding energy of each electron in the atom.
Just to get oriented, click on beryllium. The binding energies are plotted on the x-axis. Note the x-axis is
stacked up on top of itself and uses different scales (0 – 10, 10 – 100). This is done to create a ‘zoom in’
effect allowing you to clearly see the number of electrons with low binding energies. The y-axis provides
information on the number of electrons having a particular binding energy. So beryllium has 2 electrons
with a binding energy of 11.5 (units of megajoules per mole) and 2 electrons with a binding energy of
0.9.
ANALYZING SOME DATA
1. To display the first spectrum, click on hydrogen and answer the following questions:
a. First remind yourself… how many protons and electrons in a H atom?
b. How does the binding energy relate to the first ionization energy for hydrogen? Confirm your
answer by looking up the value for the first ionization energy of hydrogen. NOTE: the energy
values on the PES graph are Megajoules…
c. What TWO factors influence binding energy, for this and for any other electron in an atom?
2. We learned from first ionization energy data that the two electrons in helium are in the same shell of
the atom. BEFORE YOU CLICK on the PES data for helium, predict what you will see when you do…
what the PES data will look like.
a. How many peaks will you see?
b. Will the peak(s) be at a higher/lower/same energy value as the peak for hydrogen? Why?
c. Now click on helium and discuss what you see. Were they what you predicted?
3. Thinking about a lithium atom.
a. Again, remind yourself of a few things: How many protons? How many electrons? How
many shells? How many electrons are in each shell?
b. Now click on the element lithium and discuss how the PES data confirms these facts.
4. Skip ahead to carbon. Again, check in with a few things you should already know about carbon:
How many protons? How many electrons? How many shells? How many electrons are in each
shell? Now look at the PES data and answer these questions:
a. How many electrons in a carbon atom are positioned closest to the nucleus?
b. How many electrons are positioned on the outermost surface of the atom?
c. Is there evidence for what one might call a “subshell”?
d. Now draw a diagram that shows the carbon atom as a series of concentric arcs
representing circles (with the nucleus at the center) where each circle locates the positions
of the electrons relative to the nucleus. The first arc is drawn for you:
5. Now look at nitrogen, again thinking first about what you know of this atom… # protons, # electrons,
# of shells, and # of electrons in each of these shells. Then CLICK:
a. What similarities to carbon do you see?
b. Why are all the binding energies higher?
c. Draw a diagram (as you did above for carbon) of the nitrogen atom.
6. Repeat with oxygen, creating the same diagram. .
7. Now for something different. Compare oxygen to sulfur: what is similar; what is different; draw a
diagram for the sulfur atom.
8. Compare neon to sodium and draw diagrams for each.
9. Compare argon to potassium:
a. Does the pattern look similar to your comparison of neon and sodium?;
b. What seems to be happening in the transition between a noble gas (Ne or Ar) to the
subsequent group I metal (Na or K)?
10. Look at the binding energies for the outermost electron of Li, Na, and K. What do you notice; can
you explain the pattern?
Fill in the table on the next page and then discuss how the pattern in the binding energies relates to the
periodic table. Look carefully: there are a few things you should be able to discover.
Element
Li
Be
B
C
N
O
F
Ne
Na
Mg
Al
# e-‘s closest to
nucleus
# e-‘s farthest
from nucleus