Electron Shielding and Effective Nuclear Charge
Critical Thinking Exercise
This exercise uses an Excel spreadsheet template to predict the strength of the attractive force acting on an atom's valence electrons. The file 'effective nuclear charge sim - Part 1.xlics' is available on CANVAS within the "Exercises" module. Download and open this file in Excel to make the observations and answer the questions below.
I. Collect and plot data:
The number of protons and electrons in an atom can be set using the widgets on the left. The extent to which the outermost electron (last one added) is shielded from the positive charge of the nucleus is calculated by the spreadsheet and subtracted from the actual nuclear charge (Z, atomic number) to get the effective nuclear charge, Zeffective, that attracts the outermost valence electron.
Use the simulation to determine the Zeffective for neutral atoms of the elements sodium through calcium. Create a graph of both Z and Zeffective on the y-axis plotted vs. the number of protons (Z = 11 through 20) on the x-axis. Use the Scatter Plot graphing option for this. Your graph will have two sets of data on the same graph, one of wriit.n viii De a straight diagonal line. Be sure to label your graph appropriately and print a copy to turn in with this exercise.
An example table and graph is shown below for a series of 4 elements, lithium through carbon:
|
Atomic No.
|
Actual charge Z
|
Effective charge Zeff
|
|
3
|
3
|
1.2
|
|
4
|
4
|
1.88
|
|
5
|
5
|
2.55
|
|
6
|
6
|
3.23
|
II. Use your graph and the simulation to answer the following quesktI4nst
1) For the elements across the 3rd row of the Periodic Table (Na through Ar), the actual nuclear charge, Z, increases by 7 proton units.
a) By how much (in proton units) did the effective nuclear charge, Zeff increase) (Be sure the number of electrons is the same as the number of protons for each element so that you are comparing neutral atoms!)
b) Which element in the row exerts the greatest attraction on its valence electron(s), and which element exerts the least attraction on its valence electron(s)?
2) How does the Zeff for potassium compare with the Zeff for argon? Explain why the trend in Zeff suddenly reverses when another proton and electron are added to Ar to create a K atom.
3) Create a potassium cation, K+, by removing one electron from the potassium atom to leave it with a positive charge.
a) How does the Zeff for K+ compare with the Zeff for a neutral potassium atom? Try to explain what you observed.
b) How does the Zeff for K+ compare with the Zeff for a neutral atom of argon? Try to explain what you observed.