Electronegativity is the ability of an atom to attract electronsx to itself.

If you examine the periodic table you'll find that, with the exception of the noble gases in the far right column, electronegativity gets higher as you look to the right and up and lower as you look to the left and down.

Thus, Fluorine (F) has the highest electronegativity while Francium (Fr) has the lowest.

When atoms of different electronegativities form a bond, the electrons tend to be more controlled by the atom with the higher electronegativity.

The resulting bond is a dipole -- it has a slight negative charge on the higher electronegativity side, and a slight positive charge on the other end.

This type of bond is called a polar bond. Examples of polar bonded molecules include hydrochloric acid (HCl) and water (H₂O).

Not all molecules with polar bonds are polar molecules. To be a polar molecule, the entire molecule must be a dipole - it must have slightly positive and slightly negative ends.

Hydrochloric acid is a polar molecule, since it has positive and negative ends. Beryllium fluoride (BeF₂), however, is not a polar molecule - while containing two polar bonds, it has two positive ends. (See the illustration in the tutorial above for more details.)

When hydrogen fors a polar molecule with nitrogen, oxygen, or fluorine (all highly electronegative elements), the resulting molecule tends to be slightly positive on the hydrogen side.

The electrostatic bond between this positive hydrogen side of the molecule and other negative ions or polar molecules is called a hydrogen bond.

When atoms of similar electronegativities share electrons to form a covalent bond, the shared electrons tend to be evenly controlled by both atoms. The resulting bond is evenly charged on all sides, and is said to be a pure non-polar covalent bond.

An example of this is nitrogen gas (N₂).

Even in non-polar molecules, the fact that electrons are constantly in motion means that areas of the molecule can temporarily become slightly positive or negative.

These areas in adjacent molecules can develop weak, short-distance attractions called Van der Waals forces.

Water molecules, being polar, form hydrogen bonds with each other. Non-polar molecules present in the water, however, are not included in these hydrogen bonds. This exclusion tends to drive the non-polar molecules together, even though the non-polar molecules are not actually attracted to each other.

This phenomenon is called hydrophobic interaction. The tendency of oil to form globules in water is an example of this.

Try one of these resources to learn more about chemical bonds:

• Chemical Bonds
• Molecular Resonance and Chemical Bonding 
• Polar and Non-Polar Bonds
• Simple Ionic Bond Builder