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In our course sequence, students use VSEPR in general chemistry. Organic chemistry uses hybrid orbitals and pi-bonding. When students get to inorganic chemistry, they are often confused by the two descriptions, especially with a steric number (lone pairs plus bonded atoms) greater than four.
This activity is designed to serve as a bridge between multiple levels and returned to in more than one course.
150 different molecules can be manipulated in JSmol, with options to show multiple bonds, lone pairs, and orbitals suitable for pi-bonding.
After practicing with this material, students should be able to
a. Move between Lewis dot structures (Implementation note 1) and 3-dimensional shapes (Implementation note 2).
b. Calculate formal charges for atoms in molecules (Implementation note 3).
c. Consider how bond distances could be correlated with different resonance structures (Implementation note 4).
c. Consider which option has more reasonable formal charges (Implementation note 5).
c. Return to this resource in multiple courses (Implementation note 6).
This activity assumes students have been introduced to VSEPR theory. The goal of the activity is to provide additional practice.
1. Start with none of the view options on (default) and try drawing the Lewis dot structure on paper for the chosen molecule. Beginning students often get stuck in VSEPR problems deciding on the central atom and that is not really the interesting part. In this resource the central atom is indicated by bold type in the formula and that atom is fixed as the center of rotation.
2. Scroll down to the bottom of the page and turn on multiple bonds and lone pairs to check your Lewis dot work. Lone pairs represent two electrons (with a few exceptions indicated by non-integer lone pairs and that are shown with a single dot in the lobe.)
3. Count the formal charges by including an atom's unshared electrons and half its bond-shared electrons and compare with the atomic valence. (Pi-bonding orbitals should not be displayed when counting formal charges since those electrons will already be counted in the double bonds.)
4. If a resonance option is available, a different pi-bonding alternative is shown. This may result in decreasing the N atom steric number to give a planar geometry. Measure distances and angles to see if one resonance structure is favored.
5. If a smaller formal charges option is available, lone pairs are moved to make multiple bonds and decrease the formal charges. Count the formal charges again to verify.
6. If students have a background in pi-bonding, turn on p-pi and d-pi orbital options to show orbital alignments that could make the pi bonds. Note that these are not calculated molecular orbitals but simply atomic orbitals with the correct symmetry.
This selection of over 150 molecules has NOT been simplified for general chemistry in order to make it more interesting in the inorganic chemistry course.
a. There are three molecules with an odd number of electrons (non-integer number of lone pairs).
b. There are two molecules from Group 13 that exhibit 3-center 2-electron bonds where three atomic orbitals form three molecular orbitals (bonding, non-bonding, and antibonding) with two electrons in the bonding orbital.
c. There are three high steric number molecules where the lone-pair is not stereochemically active.
d. There are 27 molecules where the VSEPR molecular shape can be predicted without double bonds. An additional option with smaller formal charges is also given where a lone pair has been moved to a double bond.
e. There are 30 molecules where a second resonance structure is provided. Often the observed structural geometry comes from more than one resonance structure.