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Unformatted text preview: strongly: Lewis-Dot structures give bonding information, but do not accurately represent the
true shape of the molecule.
Example 3: NOH
This is a hypothetical molecule, that would be called "Nitrosyl Hydride", except that I
cannot find reference to it anywhere so I doubt that it actually exists (it more than likely
immediate breaks down to more stable compounds). We can make a Lewis-Dot structure for it,
though, whether it exists or not. The fact that it does not implies that it would be highly unstable.
None the less, let's imagine what it should look like, if, indeed, we could make it. The LewisDot structure would look like this: So, let's go through it. Central element is nitrogen; there is one lone pair and two atoms giving a
set number three. This gives us the trigonal planar parent structure, but the bent molecular
shape. It would look as follows: Dakota State University Page 108 of 232 Experiment 9: VSEPR General Chemistry I and II Lab Manual Polarity:
By now you should know how to determine whether or not a bond is polar based on
differences in Pauling's electronegativity scale (a difference in electronegativity of less than 0.5
is non-polar, between 0.5 and 1.7 is polar, and greater than 1.7 is ionic). For a diatomic
molecule, the molecule is polar if the bond is polar. This is a simple enough concept, and quite
intuitive, but what do we do with polyatomic molecules? Such a question is important since
polarity of molecules determines the intermolecular forces between such molecules, and
therefore affect many of the physical properties.
Well, for polyatomic molecules, we have a few additional steps:
1. Determine the Lewis-Dot structure.
2. Determine if a ny of the bonds are polar or not. If there are no polar bonds, you are
finished; it is a non-polar molecule. If there is even one bond, you must proceed, because it may
or may not be polar.
3. Determine the molecular shape using VSEPR.
4. The molecule is non-polar if it is highly symmetrical; otherwise, it is polar.
In a highly symmetrical molecule, the polar bonds will cancel exactly, but how can we
tell if we have a highly symmetrical molecule? Once you see it, it becomes obvious.
Unfortunately, it is often difficult to see, so let's go though these rules. A molecule is highly
symmetrical if ;
1. All elements bonded to the central atom are identical and we have no lone pairs, OR
2. All elements bonded to the central atom are identical and we have a linear molecule
with either a trigonal planar or octahedral parent shape, OR
3. All elements on diametrically opposed sides of the central atom (exact opposit sides of
the atom, so they could not "see" one another around the central element) are identical and the
chemical structure is square planar.
Dakota State University Page 109 of 232 Experiment 9: VSEPR General Chemistry I and II Lab Manual Let's see why these molecules are non-polar. The classic example is carbon dioxide,
CO2 , so let's take that one as the example. The Lewis dot structure shows that each oxygen is
double bonded to the carbon, the central element, and there are no hydrogens on the carbon.
This gives u...
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