Essay Unit 1 Lecture 6 PES Shielding Effect 2

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(PES) &
Unit 1 Lecture 6

Shell Model and the

# of
Row of Periodic electron Table & # of s it can elements in it hold n=1


1st row has 2 elements n=2


2nd row has 8 elements n=3**


3rd row has 8 elements **Learn later, the 3rd energy level can actually hold
18 electrons o Can anyone remember why?

• Inner core electrons o are in the inner shells • Valence electrons
• are in the outer most shell Spectroscopy
• In these experiments, high energy photons remove electrons from ANY shell, not just the outer shell
• The kinetic energy (KE) of the ejected electrons is determined • The frequency of the photons is recorded
• The ionization energy for any electrons is calculated using the following formula:

IE= hv- KE

Photons come from X-ray or UV

Sample is inside a vacuum sealed

In the energy
analyzer there is a magnetic field … so there is a positive and negative side
(similar to mass spec) How PES works
1. High energy photons shot at sample at a specific frequency.
2. This causes 1 electron to be released from the sample.
3. The angle that the electron strikes the electrostatic (energy) analyzer is used to determine its velocity.
Velocity is used to calculate Kinetic
(KE). just use KE= ½ mv2
- With velocity determined, IE= hvKE

• Only ONE electron is removed from each atom, but it can come from ANY shell (not just the outer)
• The data is presented from the machine as a graph…now let’s learn how to read this graph!

Reading PES data

Y axis = # of eX axis = energy (MJ/ mol)

Hydrogen… 1 e- in the n= 1 shell
Helium… 2 e- in the n=1 shell…notice the peak is 2x as tall compared to H
Lithium… 2 e- in n=1 shell and 1 e- in n=2 shell…the peak for n=1 is 2x as tall than the peak for n=2, however, the IE in n=2 is MUCH LESS, as it is further from the nucleus and is ‘shielded’ by inner core e-

energy (MJ/ mol)

• Predict what you think Beryllium might look like…
Beryllium…2 e- in in n=1 shell and 2 e- in n=2 shell
- Notice…both peaks are the same height (2 e-)

Relative # of electrons

- Takes more energy than Li to remove an e-…this is because there is less distance between e- and the nucleus

energy (MJ/ mol)

specifics of shell model • There are exceptions with Ionization Energy and trends on the P.T…. o B= 800.6 kJ/mol in the 2nd row, and is less than Be in the
3rd row (Be= 899.4 kJ/mol) o Al= 577.6 kJ/mol in 3rd row, which is less than that of the larger element in the same row, Mg= 737.7 kJ/mol

o These can be explained with PES data! Let’s discuss!

Boron’s PES
• Here we see 3 peaks…but boron is still only on the 2 nd energy level (n=2)…

Relative # of electrons

o from our data on 1st ionization energies we assumed n=2 could hold up to 8 electrons o They would have the same ionization energies, as they would be the same distance from the nucleus.

• PES data tells us that the shell model MUST BE REVISED!
• The n=2 shell must contain 2 subshells with different ionization energies. o What can you think of that you have already learned that lines up with the
“subshell” idea?

energy (MJ/ mol)

Let’s look at
• Notice there are 5 peaks…

Relative # of electrons

o 1s22s22p63s23p1





specifics of shell model The exceptions of B and Al show they require less energy to remove electrons than what we would expect in the trend because the outermost electron is in a new subshell (like a sub energy level) all on it’s own making it easier to remove

Tips for Reading PES
1. Size of peaks o Tells you the # of electrons in an energy level

2. # of peaks

Tells you the # shells (and subshells) there are

3. Values off graph

This is where you find the exact ionization energy value

4. Use your PT!

Remember the basics from electron configuration you learned and trends that would affect the amount of ionization energy

Try reading a graph with LOTS of data! • asdf