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 molecular shapes.

Linear

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.  

Trigonal Planar

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.

Tetrahedral

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 electrons.

Pyramidal

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.

Bent

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

Polarity

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.

Intermolecular Forces

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.

Dispersion:

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.

Dipole-Dipole

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

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 special properties.