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 compound (d–):

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.