Pictures of the Day CH310M/CH318M Fall 2007

9-07-07

BOND ANGLES AND SHAPES OF MOLECULES
• The Valence-Shell Electron-Pair Repulsion (VSEPR) model of molecular structure assumes that areas of valence electron density around an atom are distributed to be as far apart as possible in three-dimensional space.


- When using the VSEPR model, lone pairs of electrons, the two electrons in a single bond, the four electrons in a double bond, and the six electrons in a triple bond are each counted as only a single area of electron density.
- Four areas of electron density around an atom adopt a tetrahedral shape with bond angles near 109.5°, such as in methane, CH4.

- Three areas of electron density around an atom adopt a trigonal planar shape with bond angles near 120°, such as in formaldehyde, H2C=O.

- Two areas of electron density around an atom adopt a linear shape with bond angles near 180°, such as in acetylene or CO2.

- The VSEPR model predicts shape but does not explain why the electrons are located where they are. Thus, it is only a useful model and not a theory.

Click on each category to see animated GIFs of examples of molecules with the indicated geomtry. The final category is of a hybrid molecular containing different atomic geometries. You must be able to simply look at a molecular structure, and be able to predict the geometry around each atom using the VSEPR ideas. Later, you will learn to think of this in terms of hybridized atomic orbitals. Note that we have broken up the different categories onto different pages to avoid a very long download for a single page.

Tetrahedral Geometry (4 Areas of electron density)

Trigonal Planar Geometry (3 Areas of electron density)

Linear Geometry (2 Areas of electron density)

Hybrid Geometry (Different atom types)
Shown above are the Molecular Dipole Moments for some common molecules. Molecular dipole moments are the vector sums of the individual bond dipole moments in a molecule. Being able to predict in a qualitative way what a molecular dipole moment looks like is important for this class because it is requires the synthesis of all the ideas about electronegativity and molecular three-dimensional shapes that we have discussed thus far. 
Shown above are the most important resonance structures for a variety of common organic ions along with calculated electrostatic potential surfaces. For resonance to be important, look for a pi bond adjacent to the atom with the charge. For other rules of drawing appropriate resonance structures reread lecture notes from today. Note that the first molecule, the ethoxide anion, does not have resonance (no adjecent pi bonds), and the negative charge (red color) is localized entirely on the oxygen atom. On the other structures, the charges are more delocalized onto at least two atoms, so the colors are less intense. Delocalization of charge is stabilizing (4th Golden Rule of Chemistry). Remember that molecules are not equilibrating between the contributing structures, but rather, spend all of their time as a resonance hybrid of the two. When the resonance contributing structures are not symmetrical, there are several rules given in class today that explain which contributing structures dominate. In the case of the so-called enolate ion shown at the bottom, the major contributor is the structure on the right because it places the negative charge on the more electronegative oxygen atom. Note that in the electrostatic potential surface of the enolate ion most of the red color is located on the oxygen atom.