25 Jun 2011

Periodic trends in atomic size and electronegativity based on MO calculations

In-Class Activity

Submitted by Robert C. Scarrow, Haverford College

In Haverford College's course Chem 111:Structure and Bonding, we have included a workshop exercise that guides students through their first experience using electronic structure calculations.  We use the WebMO interface along with Gaussian03, but the exercise could be adapted for other electronic structure programs. The general structure of the exercise is as follows:

  • Each student in the class performs an MO energy calculation on an H2 molecule with a different H-H distance in the range of 0.5 to 2.0 Å.  The class data is used to construct an E vs. r graph for the H-H bond.  The students then verify that the "optimize geometry" calculation will find the minimum energy configuration of the H2 molecule.
  • The students perform similar calculations for O2 and discover that the triplet state is lower than the singlet state (in addition to learning what "singlet" and "triplet" mean).  Then using the O2 molecule they learn to visuallize the molecular orbitals and learn how the "isovalue" affects the appearance of the MOs.
  • The students each take a different HX molecule with X from 2nd or 3rd row and calculate dipole moments, atomic (Mulliken) charges, and bond distances.  Then as a class the students use their data to construct an electronegativity scale and see how their scale compares with the Pauling scale.  They also use the bond lengths to construct a table of atomic covalent radii, and discuss the periodic trends.
  • The students are given some suggestions for further topics to explore using electronic structure calculations.  Suggestions include (1) Calculating the shapes and bond angles in various XYn molecules and comparing them to VSEPR predictions.  (2) Discovering inductive effects on the partial charge on atoms depending on electronegativity of atoms several bonds away.  (3) Shapes of MOs in CO (vs. those in O2).  (4) Shapes and energies of MOs in benzene.

In the Fall of 2010 these activities all were part of a 3 hour laboratory period devoted to computational chemistry.  The report form for this activity is included as an attachment.  I've also attached a slightly modified version of the description of the exercise from the Fall 2010 lab manual for the course.  My plan for next semester is to implement this as a 1 hour recitation exercise followed by a series of homework activities.  Stay tuned.

Learning Goals: 
  • Students will learn how to carry out and interpret electronic structure calculations for real and hypothetical small molecules
  • Students will learn how to visualize molecular orbitals and electronic potential surfaces derived from electronic structure calculations
  • Students will relate results from electronic structure calculations to properties such as atomic radius and electronegativity
  • Student's prior learning about periodic trends in atomic radius and electronegativity will be reinforced
Equipment needs: 

Students can use their own laptops or institution-owned computers.  Institution supplies computer cluster operating WebMO and Gaussian software.

Time Required: 
3 hours (or 3-4 1 hour sessions)
Evaluation Methods: 

Students turned in the lab reports for grading.  Nearly all students completed the lab report - a few needed extra time after the lab period to finish up calculations. 

We also asked students their opinion of the usefulness of the MO exercise on the lab evaluation form.  I comment on these more in the next section.

Evaluation Results: 

Of the five lab exercises done during the semester (some were multi-week) the MO exercise received the lowest marks from students on the lab evaluation form.  A significant fraction of students in the class (15 out of 45) included comments about this lab that indicated that many of the students did not understand the point of the calculations or how it related to what we were discussing in the course.  As a result of these comments I will be implementing this exercise in the Fall 2011 offering of Chem 111 as part of the regular course meetings rather than the lab so it can be closely integrated with the class sessions on molecular orbitals.

Creative Commons License: 
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