Jens-Uwe Kuhn, Santa Barbara City College
Jessica Martin, Northeastern State University
Creative Commons License: Attribution-Noncommercial-Share Alike 3.0 United States License.
Trends in the Periodic Table II: Ionization Energy, Electron Affinity, and
Electronegativity
General trends in the periodic table for first and second ionization energy, electron affinity, and
electronegativity can provide further information about chemical reactivities of various species,
including transition metal complexes. The radius of transition metal ions in inorganic
coordination compounds is of great importance in many biologically relevant coordination
compounds.
Objectives:
(1) Predict trends in the periodic table for first and second ionization energy, electron affinity,
and electronegativity.
(2) Describe specific reasons for these variations across a period and within a group, including
exceptions.
A. First and Second Ionization Energy
Critical Thinking Questions:
1. Ionization energy (IE) is defined as the energy required to remove an electron from a
gaseous atom or ion. The energy required to remove the highest-energy electron of an
atom is referred to as the first ionization energy (IE1). Write out a balanced general
equation, which describes this process (use “X” as a general element).
2. The energy required to remove the second highest-energy electron of an atom is referred
to as the second ionization energy (IE2). Similar to question #1, write out a balanced
general equation, which describes this process (use “element X” again).
3. In general, would you expect values of IE1 to be larger or smaller than values of IE2 for the
same element? Explain your reasoning.
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4. Use your reasoning from question 3 to compare the difference between the first and
second ionization energy for sodium to the difference between the first and second
ionization energy for magnesium. For which element, sodium or magnesium, would you
expect this difference to be larger and why? Consider the different origin of the second
electron that is removed from sodium compared to that of the second electron being
removed from magnesium.
5. For which electrons would you expect a large increase in ionization energy, regardless of
the element’s position in the periodic table (consider your reasoning from question 4 and
formulate a general statement, which describes for the removal of which electrons you
would expect a large jump in the ionization energy, regardless of the element’s position in
the periodic table)?
6. How would you expect the first ionization energy to change as you go across a period in
the periodic table (note that this is referring to the trend of IE 1 values, not to the difference
between IE1 and IE2 values)? Consider what you have learned previously about atomic
radii and shielding of valence electrons.
7. How would you expect the first ionization energy to change as you go down a group in the
periodic table? Make your prediction based on what you have previously learned about
electron configurations and atomic radii.
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8. First ionization energies for some elements are listed in model 1 below. Use this
information to compare your predictions and statements from questions 3 – 7 to the actual
data given in the table (note that the second ionization energies for sodium and magnesium
are 4560 kJ/mol and 1445 kJ/mol, respectively). Are your predicted trends the same as
shown in the table?
Model 1: First Ionization Energies for alkali metals and elements in period 3:
IE1 (kJ/mol)1
520
495
419
409
382
735
580
780
1060
1005
1255
1527
Element
Lithium
Sodium
Potassium
Rubidium
Cesium
Magnesium
Aluminum
Silicon
Phosphorus
Sulfur
Chlorine
Argon
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Zumdahl, S. Chemical Principles, 2005,
Houghton Mifflin: Boston, MA.
B. Electron Affinity
Critical Thinking Questions:
9. Electron affinity (EA) is defined as the energy change associated with the addition of an
electron to a gaseous atom. Write out a general equation (using “element X”), which
describes this process. Compare and contrast this process to the process described in
question 1.
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10. Taking into account thermodynamic conventions, would you expect electron affinities to be
negative or positive values? Briefly explain your reasoning.
11. Predict the change in electron affinity values across a period of the periodic table. Justify
your reasoning.
12. Predict the change in electron affinity values down a group of the periodic table. Justify
your answer.
13. Some electron affinities are given in Model 2 below. Compare your predictions from
questions 10 – 12 to the given data. Are there any outliers?
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Model 2: Electron affinities for alkali metals and some elements in period 3:
EA (kJ/mol)2
- 59.6
- 52.8
- 48.4
- 46.9
- 45.5
- 42.5
- 134
- 72.2
- 200
- 349
Element
Lithium
Sodium
Potassium
Rubidium
Cesium
Aluminum
Silicon
Phosphorus
Sulfur
Chlorine
2
Lide, D.R. (ed.) CRC Handbook of Chemistry and Physics,
2001, CRC Press: New York.
Problem:
14. Electron affinity values are not available for every element, since not all elements form
stable X- ions. Suggest and discuss reasons why some elements do not form stable Xions.
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C. Electronegativity
Electronegativity is defined as the ability of an atom in a molecule to attract shared electrons to
itself. Electronegativity values are typically determined from the difference between the
measured bond and the expected bond energy of a particular bond.
Critical Thinking Question:
15. Considering what you have learned about trends in the periodic table, predict an increase
or a decrease in electronegativity values when going across a period of the periodic table,
as well as when going down a group of the periodic table. Compare these trends to your
knowledge of the most and least electronegative elements and their positions in the
periodic table.
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