Remember that the formation of a bond between two substances reduces the amount of chemical energy and the breaking of a bond increases the amount of chemical energy. A chemical reaction will produce heat if atoms rearrange themselves so that the chemical energy of the newly created bonds are lower than the chemical energy of the bonds that were broken. Covalent bonds and ionic bonds are "strong" because the unbonded atoms or ions have a much higher amount of chemical energy before forming a covalently bonded molecule or ionic compound.
Large molecules are often due to chains of similar or identical chemical units. The covalent bond holding each unit together to make a large molecule can be weaker than the covalent bonds holding the atoms within each chemical unit (for example, the bond holding together the chains of glucose to make starch), but they can also be very strong, (for example. the bond holding the nucleotide chains that make DNA). Polymers are large molecules in which chains of repeating smaller units are held together by a covalent bond. Monomers are the molecules that combine to form the units of polymers. Some polymers contain only one type of monomer, for example, polyethylene and starch but many contain monomers that belong to a group of molecules. The following groups of monomers, nucleotides and amino acids, form the following polypeptides, DNA and proteins, respectively.
In contrast to a covalent bond, a hydrogen bond is a "weak" bond, (about 5% of the strength of an average covalent bond), that is often between molecules and, (if the molecule is big enough and has the right kind of atoms), sometimes between atoms of the same molecule. These bonds are weak because the loss of chemical energy due to the formation of hydrogen bonds is small. However that does not mean that hydrogen bonds are simple and unimportant. They form the strongest intermolecular forces and they determine the properties of many substances that are necessary for living things, including: water, DNA and proteins.
Be sure to read and study all of the links in this section. Test your understanding of this section by answering the Background Worksheet.
to Polymer and Hydrogen Bonds Home Page
Back to Web Lessons Home Page
Last update: May 2000
©2000 The Exploratorium