Why do greenhouses gases need to have dipoles?

In summary, according to this chemist, a greenhouse gas requires a permanent dipole or something that can induce a temporary dipole in order to absorb and re-emit heat energy. This is why oxygen gas, which does not have a dipole moment, is not a greenhouse gas.
  • #1
FredericChopin
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Greenhouse gases absorb and then re-emit the heat energy of infrared rays which have been partially absorbed and then reflected off the Earth from the sun, which is how the greenhouse effect works. I have heard, however, that in order for a gas in the atmosphere to be considered a greenhouse gas in the first place, it must have at least a permanent dipole or something that can induce a temporary dipole.

Nobody in my chemistry class knows why and neither does my professor. We have all been looking for explanations as to why, but so far none of them are complete or satisfactory.

For example, one explanation I found was that the changing electric charge in a dipole was the necessary condition for a molecule to interact with the electric field, which is what electromagnetic waves travel in. But this isn't consistent because, for example, oxygen gas (which, although irrelevant, isn't a greenhouse gas) doesn't have any dipoles whatsoever yet it can interact with UV light to break down into oxygen radicals and produce ozone. This makes the theory I found, therefore, inconsistent.

Can anybody provide a consistent and complete explanation as to why greenhouse gases require a permanent dipole or something that can induce a temporary dipole?

Thank you.
 
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  • #2
FredericChopin said:
For example, one explanation I found was that the changing electric charge in a dipole was the necessary condition for a molecule to interact with the electric field, which is what electromagnetic waves travel in. But this isn't consistent because, for example, oxygen gas (which, although irrelevant, isn't a greenhouse gas) doesn't have any dipoles whatsoever yet it can interact with UV light to break down into oxygen radicals and produce ozone. This makes the theory I found, therefore, inconsistent.

Can anybody provide a consistent and complete explanation as to why greenhouse gases require a permanent dipole or something that can induce a temporary dipole?

Thank you.

Oxygen is a good example.
Dioxygen is not a greenhouse gas because it can only absorb ultraviolet. It cannot absorb infrared.

ALL substances can absorb ultraviolet. This is because all substances consist of electrons and nuclei which have different charges (they are held together by opposite charges). Electromagnetic field can therefore separate the charges of nuclei and electrons, causing transition dipole moment. But this is only possible if the field has sufficient frequency, compared to the orbital frequency of electrons. In substances where all electrons are very strongly held and there are no low lying excited states, only very high frequency far UV waves can be absorbed, like helium, dinitrogen, calcium fluoride etc. In substances where low lying electron excited states are available like oxygen, near UV or even visible light can be absorbed.

But this does not mean infrared can be absorbed!

Now, another possibility to oscillate is nuclei. These are much more massive than electrons, so oscillate at a low frequency. So they might absorb infrared.

But in dioxygen both oxygen atoms are equal. So even if O-O bond oscillates, there is no dipole moment. Thus dioxygen can absorb ultraviolet, but not infrared.

Infrared is absorbed mainly by compounds where dipoles can be induced. Like carbon dioxide: no dipole moment because the two C-O bonds are opposite and equal, and the dipole moments cancel. But infrared radiation can cause charge separation to move carbon in one direction and the two oxygens in opposite direction. This induces a dipole and allows absorption.
 

1. Why are greenhouse gases considered to have dipoles?

Greenhouse gases, such as carbon dioxide and methane, have dipoles because they have an unequal distribution of charge within their molecules. This means that one end of the molecule has a slightly positive charge while the other end has a slightly negative charge. This separation of charges creates a dipole moment, making the molecule polar.

2. How do dipoles in greenhouse gases contribute to the greenhouse effect?

The dipoles in greenhouse gases play a crucial role in the greenhouse effect. When sunlight enters the Earth's atmosphere, some of it gets absorbed by the Earth's surface and warms it. The warmed surface then emits infrared radiation, which is absorbed by greenhouse gases. The dipoles in these gases help to trap the infrared radiation, causing the Earth's surface and atmosphere to warm up.

3. Can non-polar molecules contribute to the greenhouse effect?

No, non-polar molecules do not contribute significantly to the greenhouse effect. This is because they do not have dipoles and therefore cannot absorb and trap infrared radiation. Non-polar molecules, such as nitrogen and oxygen, are the main components of the Earth's atmosphere and do not contribute to the warming of the Earth.

4. How do human activities impact the dipoles in greenhouse gases?

Human activities, such as burning fossil fuels and deforestation, release large amounts of greenhouse gases into the atmosphere. This increases the concentration of greenhouse gases, which leads to more dipoles being present in the atmosphere. As a result, more infrared radiation is trapped, leading to an increase in the Earth's temperature.

5. Is it possible to reduce the greenhouse effect by changing the dipoles in greenhouse gases?

No, it is not possible to change the dipoles in greenhouse gases. The dipole moments in these gases are determined by the chemical structure of the molecules and cannot be altered. However, reducing the amount of greenhouse gases in the atmosphere can help to mitigate the effects of the greenhouse effect and global warming.

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