4 Bonding and Energy Transfer
4.2 Formal Charge
4.5 Bond Order, Lengths, and Strengths
4.6 The Shape of Things - VSEPR Theory
4.8 Greenhouse Gases
4.9 Ozone Layer
4.42 Learning Outcomes
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Greenhouse gases are gases in our atmosphere (or wherever actually) that do a good job absorbing infra-red (IR) radiation. We generally think of heat as the manifestation of IR absorption. You can think of it as a way to store heat energy. Molecules that readily absorb IR radiation increase in energy through vibrational motion of the atoms in the molecule (quantum levels in vibrational modes, as opposed to electronic). The chemical bonds stretch and bend at frequencies matching the frequency of the incoming IR radiation. Crazy fact - greenhouse gases in our atmosphere get a bigger dose of energy (IR) from the earth than they do directly from the sun - about 60% more.
Yes, over half of the IR radiation that our atmosphere adsorbs is from the earth. How? Well, the majority of the suns radiation (energy-wise, as in joules) is in the visible and UV regions. Yes, there is plenty of IR too (actually more photons), but remember, there is far more energy in the visible and UV regions than in the IR. So lets figure out where all that radiation is going.
Note: All of the percentages given in the discussion below and in the diagram are percentages of the original sun's incoming radiation. Do not take those percentages of the percentages. Think of it as the original 100% coming from the sun gets divvied up in all these various percentages.
To start with, right out of the gate, our lovely atmosphere reflects a sizable chunk of the radiation from the sun right back into space. Let's talk numbers here. 25% of all the suns radiation that hits the earth is reflected back into space. This means that 75% of the radiation "gets through"... then, 23% of that is absorbed by the atmosphere directly, leaving 52% that reaches the surface of the earth. Well, earth too has a lot of reflective surfaces so about 6% of that 52% is sent right back up into space.
So less than 50% (46% to be exact) of the suns radiation is absorbed by the surface of the earth. That is ALL of the types - IR, VIS, and UV. What happens to that? Well it all gets absorbed by various substances, and thanks to energy conversion most all of the UV and VIS is converted into heat (shaking and vibrating the molecules of matter on an atomic scale) and things warm up considerably. That heat is then reemitted back up into the atmosphere, but as IR radiation (heat) as the matter relaxes back to more of a ground state (cooler temperatures). So 46% is now going back up and it is all IR radiation. The atmosphere then absorbs 37% of the 46% and only 9% escapes back into space. What does all this mean?
Well, first off, it means that our earth (thankfully) is heated quite nicely by the sun and our atmosphere really helps "keep the heat in" so to speak. If we didn't have the atmosphere with its greenhouse gases, the earth would average about –15°C. That's 5°F! Quite a bit colder than our average global temperature - which is currently about 15°C or 59°F. So this is good news for us folks who like it warmer than the inside of your freezer compartment.
Let's remember that we don't just keep absorbing heat. Go back and look - 23% of the sun's radiation is directly absorbed by the atmosphere, and another 37% is cycling into the atmosphere via the earth reemitting the heat absorbed at the surface. So we suck up 60% of the suns radiation. And yes, that 60% DOES eventually bleed back out into the universe. What this means is we are at a steady state equilibrium with heat in and heat out (heatin= heatout). The rate of heat in and heat out will determine the average temperature because of the lag in energy transfer. The heat coming in is fairly constant from the sun with little variation over 1000's of years - heck 100,000's of years. The heat bleeding out is where there is a big difference now. The heat to bleed out is going out much slower than long ago and that means the incoming heat resides here on earth longer which leads to much higher global temperatures. It is straight up science - which is awesome BTW.
So in general, a greenhouse gas has to be able to absorb IR radiation. There is a big criteria for this. An IR photon, in order to be absorbed, must have a molecule that has a disruptable charge distribution of negative vs postive charge. This means there needs to be a dipole moment available - it can be permanent, or induced temporarily. Charge distribution is always perfectly centered for all diatomic homonuclear molecules. This means that both nitrogen N2 and oxygen O2 do NOT absorb any IR radiation at all. Neither does argon - it doesn't even have a bond, its monatomic. Well, that's 99.9% of our atmosphere right there. So be thankful for that other 0.1% because that is where the greenhouse gases are. (Once again, I'm leaving out the continuously variable water vapor here which accounts between 0 and 5% of the atmosphere depending on climate)
So what are the greenhouse gases? The natural ones in order of impact/amounts are water H2O, carbon dioxide CO2, methane CH4, nitrous oxide N2O, and ozone O3. There are other manmade ones as well like chlorofluorocarbons CFCs, and hydrofluorocarbons HFCs and HCFCs.
Nice graphic and blurb about greenhouse gases on OpenStax.
And a really nice site from the American Chemical Society (ACS) : ACS Climate Science Toolkit
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