More on greenhouse gases The article explains briefly why some gases in the atmosphere absorb infrared energy and some do not. Recall from earlier in this Teacher’s Guide that the Earth absorbs and re-radiates some of the sun’s energy, and recall that the reflected energy is in the infrared range. If the only gases in the atmosphere were oxygen and nitrogen then all of the energy reflected by the Earth would travel back into space. But the atmosphere contains other gases like carbon dioxide and methane, and these gases absorb some of that heat and keep it in the atmosphere, producing the greenhouse effect. What makes carbon dioxide and methane different from oxygen and nitrogen?
According to the National Oceanic and Atmospheric Administration’s National Climatic Data Center:
Many chemical compounds present in Earth's atmosphere behave as 'greenhouse gases'. These are gases which allow direct sunlight (relative shortwave energy) to reach the Earth's surface unimpeded. As the shortwave energy (that in the visible and ultraviolet portion of the spectra) heats the surface, longer-wave (infrared) energy (heat) is reradiated to the atmosphere. Greenhouse gases absorb this energy, thereby allowing less heat to escape back to space, and 'trapping' it in the lower atmosphere. Many greenhouse gases occur naturally in the atmosphere, such as carbon dioxide, methane, water vapor, and nitrous oxide, while others are synthetic. Those that are man-made include the chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs) and Perfluorocarbons (PFCs), as well as sulfur hexafluoride (SF6). Atmospheric concentrations of both the natural and man-made gases have been rising over the last few centuries due to the industrial revolution. As the global population has increased and our reliance on fossil fuels (such as coal, oil and natural gas) has been firmly solidified, so emissions of these gases have risen. While gases such as carbon dioxide occur naturally in the atmosphere, through our interference with the carbon cycle (through burning forest lands, or mining and burning coal), we artificially move carbon from solid storage to its gaseous state, thereby increasing atmospheric concentrations.
The difference at the molecular level is in the type and arrangement of bonds in each molecule. Your students will likely know that both oxygen and nitrogen, the atmospheric gases with the highest concentrations, are made up of diatomic molecules, O2 and N2. Recall that because of the symmetry of the two molecules, neither molecule is polar. In fact, because the two atoms in these two molecules are identical, there can be no dipole. In fact, only gas molecules with at least three atoms have the potential to be greenhouse gases.
A word about dipole moments—the bonds within a molecule consist of pairs of electrons. The arrangement of the bonds determines the distribution of electrons, or in other words, the distribution of negative charges in the molecule. If the charges are distributed in such way as to cancel each other out, the molecule has no electric dipole. That is, the molecule is not polar—it has no permanent dipole. For example, in the BeF2 molecule which is linear in shape the two fluorine atoms are positioned on opposite sides of the beryllium. Each of the fluorine ions has a negative charge, but since they are directly opposite each other, they cancel each other and the molecule is nonpolar.
Likewise, in a CCl4 molecule there are four chlorine atoms arranged in a tetrahedral shape around the central carbon. Each of the chlorine atoms is more electronegative than the carbon, so each of the individual bonds in the molecule is polar, but the arrangement of the bonds are symmetrical so the total molecule is not polar. It does not have a molecular dipole.
As a third example, in a water molecule the two hydrogen atoms are on opposite sides of the central oxygen, but at an angle to each other. Since the oxygen is more electronegative it tends to have a more negative charge (because the electrons that form the bond are attracted more by the positive oxygen nucleus than by the hydrogen nuclei), and the two hydrogen atoms have a more positive charge. But since the two hydrogen atoms are arranged at an angle of 104.5 degrees to each other, there is a net negative charge in the direction of the oxygen and a net positive charge in the direction of the hydrogen atoms. That is, the water molecule has a dipole and is considered a polar molecule.
In the water molecule described above the molecule has a permanent dipole. But it is possible to create a temporary dipole in a normally nonpolar molecule. Recall from kinetic theory that there are three types of molecular motion—translation, rotation and vibration. Adding energy—like the heat reflected from the Earth into the atmosphere—to a molecule will cause the translational motion to increase. So the thermal IR radiation (heat) reflected from the Earth does, in fact, cause all the molecules in the atmosphere to move a little faster. As described above, this is important because this increased motion raises the temperature of the atmosphere sufficiently to enable life on Earth. Without this added energy the temperature of the atmosphere would be too cold for us to survive.
However, the heat reflected from the Earth also causes some molecules to vibrate and rotate more than normal and more than other molecules. This occurs because the frequency of the normal molecular vibrations corresponds to the frequency of thermal IR radiation. As the molecules absorb this energy the pattern of electron distribution in the bonds changes. Bonds within the molecule may vibrate by stretching and recoiling like a rubber band or the bonds may be bent. As a result a temporary dipole is created in some molecules, and any molecule in which these changes occur vibrates or rotates a little more than normal.
For example, according to the American Chemical Society’s “Climate Science Toolkit”, in the carbon dioxide molecule the “central carbon atom is ... slightly positive. ... Since the molecule is linear with equal bond lengths, the center of negative charge and the center of positive charge coincide at the central point, the carbon atom, and the molecule has no permanent dipole moment. The symmetrical stretching vibration, top representation, does not change this symmetry, does not change the dipole moment, and does not lead to IR absorption. The molecular bending vibrations ... displace the negative charges away from the line of centers of the molecule and create a structure with a dipole moment. This increased energy, in turn, is transferred to other molecules with which it collides, thus raising the temperature of the atmosphere. These kinds of gases are greenhouse gases. For much more detail on this warming mechanism see the American Chemical Society’s “Climate Science Toolkit” at http://www.acs.org/content/acs/en/climatescience/greenhousegases.html.
As noted above, the main greenhouse gases are carbon dioxide (CO2), methane (CH4), water vapor, nitrous oxide (N2O). Also contributing to global warming are ozone and CFCs. Each of the gases, CFCs excepted, occurs naturally in the atmosphere. All are present in the atmosphere but at very different concentrations: