Molecular Substances

  1. When two or more atoms are bonded together with covalent bonds a molecule is formed.
  2. Molecules are primarily formed from the nonmetal elements, because these elements have high ionization energy and high electronegativity, causing them to share electrons to form a covalent bond instead of giving them up to other atoms which would form an ionic bond.
  3. When electrons are shared between specific atoms, covalent bonds link atoms together to form a molecule. See the molecule of Vitamin C below:
    Vitamin C
  4. Grab the molecule above and drag it around.
  5. Notice that some atoms above are bonded to one, two, three, or four other atoms and that some are double bonded and some are single bonded.
  6. To understand why we need to look at the valence electrons once again.
    1. The red atoms above are oxygen atoms with 6 valence electrons.
      O = 1s22s22p4 -->
    2. In order for the oxygen to have the stable 8 valence electron structure it must share two electrons with other atoms. In other words it must form two covalent bonds. Look at the red atoms above. You should notice that they always form two covalent bonds (either two single covalent bonds or one double covalent bonds).
    3. The gray atoms above are carbon atoms with 4 valence electrons.
      C = 1s22s22p2 -->
    4. In order for carbon to have the stable 8 valence electron structure it must share four electrons with other atoms. Look at the gray atoms above to see if they always form four bonds.
    5. The white atoms are hydrogen atoms with one valence electron. Hydrogen is closest to the Nobel Gas, Helium which only has 2 valence electrons. Therefore, hydrogen will be stable if it can share enough electrons to get two.
      H = 1s1 -->
    6. Notice that all the hydrogen atoms only form one bond above.
  7. Notice that the Lewis Dot diagrams for each of the atoms shows you how many bonds will form. Each unpaired electron will form a bond with another atom.
  8. Using a piece of software called "eChem", you can experiment with how various nonmetal atoms will bond covalently to form molecules. You can download "eChem" here: http://www.investigationstation.org/sciencelaboratory/echem/download.html
  9. Molecules tend to fall into 4 broad categories:
    1. Small molecules
      1. These molecules consist of a small number of atoms strongly bonded together.
      2. Most room temperature liquids, and gasses consist of small molecules.
      3. Some examples include: water, ammonia, butane, gasoline, air (nitrogen and oxygen)
    2. Large molecules
      1. These molecules consists of a large number of atoms strongly bonded together.
      2. Many biologically important substances consist of large molecules.
      3. Some examples include: vitamins, hormones, various cellular signaling molecules
    3. Polymers
      1. These molecules consist of repeating small molecules bonded together to form larger molecules.
      2. Polymers are also large molecules, but they can be much larger than some of the large molecules listed above.
      3. Some examples include: plastic, wood, DNA, proteins, enzymes (a type of protein with a special function).
    4. Network molecules
      1. All previous examples involved molecules that were somewhat linear or sequential, with one atom bonded to the next and so on. Sometimes a network of bonds can form between many neighboring atoms.
      2. Network molecules tend to have great relative strength because of the many covalent bonds connecting neighboring atoms. Some newly created molecules of this type are promising to revolutionize everything from drug delivery, to computer processing power.
      3. Some examples include: diamond, buckyballs, and carbon nanotubes.

        To see examples of the above molecule types, go to the Melange of Molecules web pages. Just close the window of molecules when you are done.

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