- Often when atoms share electrons in a covalent bond, the sharing
is not equal. The electrons tend to aggregate nearer to one atom
than to the other atom.
- This creates what is known as a dipole. In a molecule with
a dipole, one end has a higher concentration of electrons than the
other end. The end with more electrons has a partial negative charge
while the other end has a partial positive charge. Water, as will be
discussed below, has a strong dipole.
- A hydrogen bond is formed when the negative end of one
molecule becomes oriented to and semi-attached to the positive end
of another. Hydrogen bonds are fairly weak, but many of them in
series, as seen in DNA, can be quite strong.
- Molecules with a dipole are known as polar or hydrophilic
molecules, those without a dipole are known as non-polar or hydrophobic