10/7/13-10/11/13

This week in AP Chemistry we continued work on Lewis structures and VSEPR theory. Additionally, we learned the concepts such as formal charge and polarity.

Near the beginning of the week, we finished up the balloon/gumdrop activity, which pertained to VSEPR theory. While Lewis structures help to understand the composition and covalent bonds of molecules, VSEPR theory (short for "valence shell electron pair repulsion") allows us to predict the shape of molecules. There are two main categories in VSEPR theory: electron domain geometry and molecular domain geometry. In predicting a molecules electron domain geometry, we assume that electron pairs are placed as far apart as possible. Electron pairs are referred to as electron domains; one electron pair is equal to one electron domain. Double and triple bonds also only count as one electron domain. To determine electron domain geometry, one first counts the number of electron domains and then chooses the corresponding shape. Pictured below are several electron domain geometries.  

Different electron domain geometries. For two electron domains, the shape is linear. For three domains, the shape is trigonal planar. For four domains,  the shape is tetrahedral. For five domains, the shape is trigonal bipyramidal. For six electron domains, the shape is octahedral.  

Molecular domain geometry is slightly different. It is defined by the positions of only the atoms and not the nonbonding electron pairs. Here are some examples of molecular geometries:

When there are two electron domains, the following molecular geometries are possible:

• AB2 - linear

When there are three electron domains, the following molecular geometries are possible:

• AB3 - trigonal planar
• AB2E - bent
• ABE3 - linear (this is a rare occurrence)

When there are four electron domains, the following molecular geometries are possible:

• AB4 - tetrahedral
• AB3E - trigonal pyramidal
• AB2E3 - bent
• ABE3 - linear

When there are five electron domains, the following molecular geometries are possible:

• AB5 - trigonal bipyramidal
• AB4E - seesaw
• AB3E2 - t-shaped
• AB2E3 - linear

When there are six electron domains, the following molecular geometries are possible:

• AB6 - octahedral
• AB5E - square pyramidal
• AB4E2 - square planar

To help learn the concept of molecular and electron domain geometry, I completed a POGIL in class and made models of molecules using gumdrops and toothpicks. I also watched a lecture on VSEPR theory. While I feel I have a decent understanding of this subject, I feel I could use more practice determining the molecular geometry and electron domain geometry. I sometimes get confused with differentiating between the two kinds of geometries, as the electron domain geometry involves lone pairs of electrons and the molecular geometry does not.

We also covered the concept of formal charge in class.  Formal charge is equal to the number of valence electrons in a free atom minus the number of bonds plus the number of nonbonding electrons. The sum of each individual formal charge of each atom in a molecule must be equivalent to the total charge of the molecule. If there are multiple structures for a molecule, the molecule with the lowest formal charge is preferred. If there is a negative formal charge, it should be on the least electronegative atom if there is a choice. If the central atom is in period three or higher, multiple bonds are possible if it will reduce the formal charge of the molecule. To help me learn formal charge, I had to view a lecture on formal charge. In class, I completed the Lewis Structures III POGIL and the Lewis Structures IV POGIL, both of which dealt with formal charge. I felt I had a relatively strong grasp of this concept, although I sometimes have trouble finding which Lewis structure has the lowest formal charge when there is more than one possible structure.

Finally, we touched on the concept of polarity in a lecture that was assigned last week. An molecule is polar when its electrons are not shared equally. Shared electron pairs in polar covalent bonds are not shared equally, while electron pairs in nonpolar covalent bonds are shared equally. Between the atoms of molecule, the greater the difference in electronegativity, the more polar the bond. Molecular polarity is possible and is calculated by adding up the individual bond dipoles. One thing I did not understand about polarity is the "dipole moment." From the lecture and from various sources online I was unable to gather a clear definition. I know it pertains to electrical charge and electrons, but I feel I need a concrete definition in my head to best understand what a dipole moment is.