Absolutely Small - Michael D. Fayer [139]
Water does not absorb the Earth’s black body radiation at the peak of the black body spectrum where a lot of energy is emitted. However, carbon dioxide does. As we discussed in Chapter 17, molecules have quantized vibrational energy levels. Carbon dioxide, CO2, is made up of three atoms, with the carbon in the center. It is a linear molecule that can undergo bending vibrations. The vibrational motion has quantized energy levels. By happenstance, the energy difference between two CO2 bending vibrational energy levels falls at the energy of the peak of the Earth’s black body light emission. Therefore, the CO2 molecules in the air absorb a significant part of the Earth’s radiated black body energy that would otherwise go into space. The more CO2 in the air, the less radiated energy escapes the Earth’s atmosphere. The result is that as the amount of CO2 in the air increases, more and more of the Earth’s heat is trapped in the atmosphere, and the planet warms. CO2 is a greenhouse gas because of the two quantum phenomena, black body radiation and quantized vibrational energy levels.
VERY HOT OBJECTS GIVE OFF VISIBLE BLACK BODY RADIATION
While we are on black body radiation, now you know that whenever you see something glowing red, like molten lava coming out of a volcano or the hot element in an electric stove, you are seeing black body radiation. When you turn an electric stove on low, the temperature is low enough that all of the black body radiation is emitted in the infrared, and you can’t see it with your eyes. If you used a spectrometer and an infrared detector, you could measure the IR colors emitted. The spectrum of the IR black body radiation from the stove element would tell you the temperature. When you turn the stove to high, the element turns red because it is much hotter. Most of the black body radiation is still in the IR, but the high-energy portion of the black body spectrum is in the low-energy portion of the visible spectrum, that is, it is red.
ELECTRICAL HEATING IS A QUANTUM PHENOMENON
But why does the stove element get hot at all when electricity is passed through it? In spite of the fact that the stove element itself is a large object, we saw in Chapter 19 that electrical conductivity and electrical resistance, which produces the heat, are consequences of fundamental quantum effects. Metal crystals, such as sodium or copper, have electrons in atomic orbitals that interact with each other. The atomic orbitals from the atoms in the entire crystal combine to make molecular orbitals that spatially span the crystal. Like the aromatic molecule benzene that has six electrons in six delocalized molecular orbitals formed from interacting carbon p orbitals (Chapter 18), the electrons are not associated with a particular atom or pair of atoms. Rather, the MOs span the system, and the electrons are free to roam about the entire system, that is, a benzene molecule or a metal crystal. For benzene, six interacting atomic orbitals give rise to six molecular orbitals that are delocalized over the molecule. In benzene, with only six MOs, the energy spacing between the MOs is large. In even a very small metal crystal, there are billions and billions of atoms that give rise to billions and billions of MOs. Because there are so many MOs, they are very closely spaced. In a metal, all of these MOs form a band of quantum energy states called the conduction band. Each of these MOs is spread over the entire crystal. However, we