When elements bond to form new compounds,
those compounds exist as certain geometric shapes. The shape of the
compound is determined by the Valence Shell Electron Pair Repulsion Theory (VSEPR).
According to the VSEPR theory, in small molecules, valence electrons are spaced
as far apart as possible. In molecules, shared electrons are usually found
in pairs. The shape, therefore, is determined by the valence electrons of
the central atom and the particular atoms to which it is bonded. The
diagram below provides a clearer explanation of this phenomenon.
Since all electrons are negatively
charged, they repel one another. In the BF3 compound, there are
3 bonds; each bond represents 1 shared pair of electrons. Due to the fact
that there are 3 bonds, the farthest each pair of electrons can be from another
pair is 120º. Here are the five most common
Linear molecules are arranged in one
strait line. Whenever there is a bond between only 2 atoms, the shape is
linear because it is the only arrangement possible. Linear bonds can also
exist between 3 or more atoms. In order for a molecule with 3 atoms to be linear, the central atom must have
either zero or three lone pairs of electrons. The bond angle for linear molecules is 180º.
A trigonal planar molecule is in the shape
of a triangle, each molecule being spaced 120º apart (such
as the BF3 molecule mentioned earlier). A
trigonal planar molecule consists of 4 atoms with the central atom having no lone pairs of electrons.
A tetrahedral is a three dimensional shape
where each atom is spaces 109.5º apart. A tetrahedral
molecule is comprised of 5 atoms with the central atom having no lone pairs of
Molecules that are described as pyramidal
have 4 atoms spaced 107º apart. In order for a
molecule to be classified as pyramidal it must have 3 bonded pairs and 1 lone
pair of electrons.
If a molecule has 2 bonded pairs and 2
lone pairs of electrons, it exists in a bent shape. In a bent-shaped
molecule, the atoms are 105º from one another.
Other Common Molecular Shapes
If a molecule contains only nonpolar bonds
(such as H2 and O2), the molecule itself will be nonpolar.
A compound like HF, on the other hand, would be polar because it has a polar
bond due to the difference in electronegativity between the two atoms.
(See Case 2
for more information on electronegativity.)
When determining if a molecule is polar,
you have to look at more than polarity in the bonds. The shape of the
molecule and the polarity of the bonds together determine whether the
molecule is polar or nonpolar. Molecules classified as bent, T-shaped, and
pyramidal are polar due to the arrangement of their unshared pairs of electrons;
linear, trigonal planer, square planar, trigonal bipyramidal, tetrahedral, and
ocrahedral-shaped molecules can be nonpolar if the central atom is surrounded by
only one type of atom.
The polarity of a molecule affects how it
interacts with other molecules. The positive side of one molecule is
attracted to the negative side of another. There are different types of
intermolecular forces, each with a different strength.
An atom by itself does not have a separate
positive and negative end. This is as long as the atom is symmetrical and
the electrons are uniformly distributed around the nucleus. Yet, at any
time the atom may lose its spherical shape and become a temporarily dipolar
atom. At that point, the atom has a clear positive and negative end. The
positive pole of that atom will attract the negative pole of another atom and
pull them closer together.
That same principle also applies to
nonpolar, symmetrical molecules. The larger a molecule is, the greater its
dispersion forces are because with more electrons, there can be a greater shift
in the flow of electrons. Dispersion forces, however, are the weakest of
the intermolecular forces.
While dispersion forces rely on
temporarily induced dipoles, some molecules have permanent dipoles. These
molecules are polar molecules. Polar molecules have permanent positive and
negative ends. The attraction between molecules with dipole-dipole forces
works basically the same as dispersion forces with the positive end of one
molecule attracting the negative end of another.
Since the dipole-dipole forces between
polar molecules draw them very close together, they are more likely to be found
in the liquid or solid phase than nonpolar molecules.
Hydrogen bonding is an unusual kind of
intermolecular force. It is a force only found in bonds of H-N, H-F, and
H-O. Nitrogen, Fluorine, and Oxygen are extremely electronegative while
Hydrogen is the least electronegative element. Thus, the bonds that they
form are very polar. Although Hydrogen bonding is similar to
dipole-dipole, the intermolecular force is so great that it is given its own
name. Also, in spite of the fact that this intermolecular force is called
"Hydrogen bonding," it is not actually a bond; but it is almost as strong as one.
Hydrogen bonding is responsible for giving
H2O many of its unique properties. H2O has a bent
shape. Therefore, it has 2 lone pairs of electrons. The Hydrogen
bonding property of H2O draws electrons of the Hydrogen atoms very
close to the Oxygen atom. This creates 2 extreme poles. The extreme
poles allow water to be liquid at room temperature and give it many of its other