The connections between the atoms in a compound are called
chemical bonds. Atoms form bonds by sharing their electrons with
each other, relying on the power of electric charge to keep themselves
attached. Molecules and compounds can also bond with each other. Important
bonds between atoms are covalent and ionic bonds. Bonds between
molecules or compounds are called dipole-dipole bonds.
Bonds formed through the more or less equal sharing of
electrons between atoms are known as covalent bonds.
If the electrons in a covalent bond are shared equally,
the resulting bond is called a nonpolar covalent
bond. When one atom pulls the shared electrons toward itself
a little more tightly than the other, the resulting covalent bond
is said to be a polar bond. In a polar bond, the atom
that pulls electrons toward itself gains a slight negative charge
(because electrons have a negative charge). Since the other atom
partially loses an electron, it gains a slight positive charge.
For example, the atoms in water form polar bonds because oxygen,
which has eight protons in its nucleus, has a greater pull on electrons
than hydrogen, which has only one proton.
Polar covalent bonds involve the unequal sharing of electrons.
This inequality is brought to an extreme in a bonding arrangement
called an ionic bond. In an ionic bond, one atom pulls the shared
electrons away from the other atom entirely. Ionic bonds are stronger
than polar bonds.
One example of ionic bonding is the reaction between sodium
(Na) and chlorine (Cl)
to form table salt (NaCl). The chlorine atom
steals an electron from the sodium atom. Because it loses an electron,
the sodium atom develops a charge of +1. The chlorine atom has a
charge of –1, since it gained an electron.
As seen in polar covalent compounds, due to the unequal
sharing of electrons, some molecules have a slightly positive and
a slightly negative end to them, or a dipole (di-pole = two magnetic
poles). These compounds can form weak bonds with one another without
combining together completely to create new compounds. This type
of bonding, known as dipole-dipole interaction, takes places when
the positively charged end of one polar covalent compound (d+)
comes in contact with the negatively charged end of another polar covalent
Dipole-dipole interactions are much weaker than the bonds
within molecules, but they play a very important role in the chemistry
of life. Perhaps the most important dipole-dipole bond in biochemistry
(and on the SAT II Biology) is the dipole-dipole interaction between
positively charged hydrogen molecules and negatively charged oxygen
molecules. This reaction is so important, it gets its own special
name: hydrogen bond. These bonds account for many of
the exceptional properties of water and have important effects on
the structure of proteins and DNA.