Ozone

(Fruit can easily be substituted if the use of Styrofoam raises environmental concerns.)

Assemble class into groups of three to four students for this three-part activity. Label the Styrofoam balls in the following fashion: 1 carbon, 3 chlorine, 1 fluorine, and 5 oxygen. Remember that atoms are the smallest pure unit in chemistry, and molecules are two or more atoms connected together.

How ozone is produced:
Take 4 of the oxygen atoms and connect them with a toothpick to form 2 oxygen molecules (use two toothpicks and put two oxygen atoms on each one). Take the red pencil and point it to the "bond" in one of the oxygen molecules. The red pencil simulates high-energy radiation, which breaks the chemical bonding in molecular oxygen. The model which is struck by radiation should be broken up into two oxygen atoms. One of these free atoms of oxygen should then be attached to the molecular oxygen to make an ozone molecule.

How ozone protects:
Take the purple pencil to simulate radiation hitting the ozone molecule model. This radiation, when absorbed, will cause an oxygen atom to split from the model (remove one oxygen atom). The free atom can then bond with another oxygen atom to form a molecule of oxygen or it could strike an ozone molecule, break it up, and form two oxygen molecules.

How CFCs destroy ozone:
Build a CFC molecule by connecting 3 chlorine atoms and the fluorine atom to the carbon atom. Strike one of the carbon-chlorine bonds with the "ultraviolet radiation" pencil to break the bond. The free chlorine atom can strike an ozone molecule, breaking it down to an oxygen molecule and a chlorine monoxide molecule (OCl). If a free oxygen atom hits the chlorine monoxide, the chlorine can be liberated and an oxygen molecule will be formed. The chlorine atom is then able to strike another ozone molecule.

Q: Look at the image of the hole in the ozone layer. Why did the ozone hole shrink and grow? Is the size of the hole increasing or decreasing today?
A: The hole cycles through periods of thinness and thickness of ozone (measured in Dobson units [DU]). The hole is known to be thinnest during the colder winter months (yes, Antarctica does have a winter) and thickest during the summer months. Measurements taken in some summer months have shown ozone levels over the southern ice cap to be well in excess of 300 DU, a full 100 DU more than the levels recorded in 1996, when the hole reached record size.

Q: Do volcanoes harm the ozone layer? Does chlorine from the ocean harm it?
A: Volcanoes do produce chemicals which break down ozone. However, volcanic eruptions do not have the power to send these chemicals into the upper atmosphere, and the sheer mass of many of these chemicals causes them to fall quickly from air currents to the Earth.
Ocean chlorine tends to stay in the ocean. If the chlorine-bearing water evaporates, rock salt (simple sodium chloride [NaCl]) will form. The oceans have been around since the ozone layer formed, which would seem to indicate that it certainly does not hinder the ozone equilibrium in the atmosphere.

"Hole" refers to a thinning in the ozone layer, not the total absence of ozone over an entire region.

Ozone is an unstable molecule which can easily be destroyed in the presence of CFCs (chlorofluorocarbons).

The Montreal Protocol is a treaty designed to stop the production of CFCs by 1999.

Dobson unit: (DU) a convenient scale for measuring the amount of ozone in the atmosphere.

CFC: (chlorofluorocarbon) man-made chemical used in aerosol propellants, cleaning agents, refrigerants, foams, and electronic parts manufacturing.