CHEM 1151K
October 5, 2004
Geometry - continued
Example: methane (main component of natural gas), CH4
Here, there are four electron clouds around the central carbon atom. VSEPR tells us that
these clouds repulse to get as far apart as possible. This spaces the hydrogens evenly
around the carbon atom in three dimensions. All H -C - H bond angles are 109.5o.
Below is a "wedge-dot" diagram of methane. It's more of an artist's rendering to give you
the 3-d shape. This geometry is called tetrahedral.
The name "tetrahedral" comes from the polyhedron shape you'd get if you connected the
H's by lines. Four equilateral triangles can be put together to make a tetrahedron. Here,
the carbon is in the center of the tetrahedron.
Example: Ammonia, NH3, whose Lewis structure is:
Here, the central atom, nitrogen, has 4 electron clouds around it just like the carbon in
methane. The difference is that one of these clouds is a lone pair of electrons. All 4
clouds still repulse each other and try to get as far as they can. H-N-H bond angles are
nearly (but not quite) the same as the tetrahedral bond angles in methane. Since only 3 of
the 4 electrons clouds around the nitrogen are bonding clouds, the overall geometry of the
molecule has a different name (geometry names refer to the geometries of atoms, not
electron clouds). In this case, it is pyramidal.
At room temperature, it's interesting to note that ammonia flips back and forth like an
umbrella in the wind.
The last example is water, H2O whose Lewis structure is:
Just like methane and ammonia, the central atom, oxygen has 4 electron clouds around it.
Now the difference is that two of these clouds are lone pairs. Even so, all 4 clouds
repulse and try to get as far apart as possible. The H-O-H bond angle is similar (actually
a bit smaller) to the tetrahedral bond angles in methane. The geometry is names for the
geometry adopted by the atoms and water is bent.
O
H
H
Polarity of Whole Molecules
The first thing is that you must have at least one polar bond for a molecule to be polar
overall.
If a molecule contains only non-polar bonds, the whole molecule is non-polar.
Sometimes a molecule can have polar bonds, but they cancel each other out due to their
geometry and the molecule is non-polar overall. An example of this is carbon dioxide.
Here the individual bonds between carbon and oxygen are polar. The oxygen ends are
partially negative, the carbon is partially positive. But CO2 has a linear geometry and the
polarity of one bond is exactly cancelled out by the other.
Water, however, is polar. Each H-O bond is polar, and they are not able to cancel by the
geometry. On the diagrams below, the bond polarities left and right do cancel, but they
add together in the up direction. Water is polar. The hydrogen "end" is slightly
positively charged and the oxygen "end" is slightly negatively charged.
We can use some simple guidelines for most molecules to decide if they are polar or
nonpolar. These simplified rules don't always work, but for the cases we'll examine, they
will. Keep in mind that we are only going to examine structures where there is a single
central atom to which the outside atoms are bonded.
1. If there are no polar bonds in the molecule, it is nonpolar.
2. If polar bonds are present, the molecule may or may not be polar.
a. If all the outside atoms are alike and there are no lone pairs of electrons on the
central atom, the molecule is nonpolar.
b. The molecule will usually be polar if either one or both of these apply:
- the outside atoms are not all alike
- there are lone pairs of electrons on the central atom
Note: The lab manual table on polarity does not address the issue of the outside atoms
not all being alike.
Example: Both CO2 and HCN are linear. Both contain polar bonds. Only HCN is polar
because the outside atoms are not alike (H and N; C is the central atom).
Water is polar even though the outside atoms are alike, because there are lone pairs on
the central atom and give it the bent geometry shown above.