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.

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 IE1 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.

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:

Element / IE1 (kJ/mol)1
Lithium / 520
Sodium / 495
Potassium / 419
Rubidium / 409
Cesium / 382
Magnesium / 735
Aluminum / 580
Silicon / 780
Phosphorus / 1060
Sulfur / 1005
Chlorine / 1255
Argon / 1527

1 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.

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?

Model 2: Electron affinities for alkali metals and some elements in period 3:

Element / EA (kJ/mol)2
Lithium / - 59.6
Sodium / - 52.8
Potassium / - 48.4
Rubidium / - 46.9
Cesium / - 45.5
Aluminum / - 42.5
Silicon / - 134
Phosphorus / - 72.2
Sulfur / - 200
Chlorine / - 349

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 X- ions.

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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