The Nature of Atomic Orbitals in the Bromide Anion

It is easy to use Spartan to display atomic orbitals. This makes an excellent lecture demonstration or computational laboratory for general chemistry. Create a single bromine atom on the screen by selecting Br in the entry or expert builder and then deleting the attached hydrogen. Set up a job for a single point HF/3-21G(*) calculation with a charge of -1(anion) and a multiplicity of 1(singlet).


Next, select the option to print Orbitals & Energies in your output and click on OK.

Bromide has 36 electrons, so you will need to set up 18 orbitals. Set up the surfaces menu to calculate the HOMO, HOMO-1, etc., through HOMO-17 using the high resolution mode for best results(Note:Higher resolution surfaces will increase the calculation time significantly). Submit the calculation after providing a name for the file.

Look at the output file created by this job. The orbital energies and degeneracies correspond nicely to what you should expect for s, p, and d orbitals. For example, there are five degenerate 3d atomic orbitals.


Now, visualize each of the orbitals in turn. What do you see about the size and shape of the orbitals? Compare the 3s, 2s, and 1s orbitals. Remember that red and blue are arbitrary colors to represent the orbital phase, which can be altered by opening the display properties window, clicking on the surface being displayed, and then clicking the swap phase button. If you look carefully at the 3p orbital (a mesh view is good for this) you will see that the nodal properties expected for this orbital. The radial distribution has a node at the nucleus and another farther away.

Perform similar calculations for fluoride and chloride at the same level of theory. What trends are apparent in the orbital energies?