Aha! Chemistry with Prof Bob
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    • Table of contents
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  • LEARNING MODULES
    • Chapter 02 Stuff, matter: What is it? >
      • 0200 Stuff, matter: A theory of atoms
      • 0201 Atoms: The building blocks of all stuff
      • 0202 People classifying stuffs. Why?
    • Chapter 05 Chemical reactions, chemical equations >
      • 0500 Chemical reactions vs. chemical equations. Overview
      • 0501 Chemical amount and its unit of measurement, mole
      • 0502 The Avogadro constant: How many is that?
      • 0503 The Avogadro constant: Why is it that number?
      • 0504 Chemical formulas: What can they tell us??
      • 0505 Chemical equations: What can they tell us?
      • 0506 Limiting reactants: How much reaction can happen?
      • 0507 Balanced chemical equations: What are they?
      • 0508 Chemical reactions as competitions
    • Chapter 09 Aqueous solutions >
      • 0901 What is a solution? And what is not?
      • 0902 Miscibility of liquids in each other
      • 0903 Like dissolves like? Shades of grey
      • 0905 Dissolution of ionic salts in water: A competition
      • 0906 Can we predict solubilities of salts?
      • 0907 Solution concentration
      • 0908 Chemical species, speciation in aqueous solution
      • 0909 Solutes: Electrolytes or non-electrolytes?
      • 0910 Electrolytes - strong or weak?
      • 0911 Concentrated, dilute, strong, weak
      • 0912 Species concentration vs. solution concentration
      • 0913 Weak electrolytes: Getting quantitative
    • Chapter 11: Dynamic chemical equilibrium >
      • 1100 Dynamic chemical equilibrium: Overview
      • 1101 Visualising dynamic chemical equilibrium
      • 1102 The jargon of chemical equilibrium
      • 1103 Equilibrium constants: The law of equilibrium
      • 1104 The law of equilibrium: an analogy
    • Chapter 22 Spectroscopy >
      • 2200 Spectroscopy: Overview and preview
      • 2201 Quantisation of forms of energy
      • 2202 Light: Wave-particle "duality"
      • 2203 Ultraviolet-visible spectroscopy
      • 2204 Beer’s law: How much light is transmitted?
    • Chapter 27 The greenhouse effect, climate change >
      • 2700 The greenhouse effect: overview
      • 2701 Is Earth in energy balance?
      • 2702 CO2 in the atmosphere before 1800
      • 2703 So little CO2! Pffft?
      • 2704 Does CO2 affect Earth's energy balance?
      • 2705 The "greenhouse effect"
      • 2706 Why does CO2 absorb radiation from Earth?
      • 2707 The "enhanced greenhouse effect"
      • 2708 Why doesn't CO2 absorb the radiation from the sun?
      • 2709 Why are N2 and O2 not greenhouse gases?
      • 2710 Doesn't water vapour absorb all the IR?
      • 2711 Carbon dioxide from our cars
      • 2712 The source of energy from combustion
      • 2713 Comparing fuels as energy sources
      • 2714 Methane: How does it compare as a GHG?
      • 2715 Different sorts of pollution of the atmosphere
      • 2716 "Acidification" of seawater
    • Chapter 27 Communicating chemistry >
      • 2700 Overview, preview
      • 2703 The jargon we use
  • TEACHERS' CORNER
    • T01 Communicating chemistry
    • T02 Beer's law
    • T03 Professional amnesia of the chemistry teaching professio
    • T04 Law of equilibrium
    • T05 Visusalizing dynamic chemical equilibrium
    • Information vs. knowledge
  • PERSONAL GALLERY
    • Family
    • Travel
    • Playful dolphins
    • The University of Western Australia
    • Kings Park
    • Perth
    • At work
    • 999 Thermodynamics
Module 2709
​

Why are nitrogen and oxygen not greenhouse gases?

The majority of molecules in the atmosphere are nitrogen and oxygen molecules. Why do they not absorb radiation emitted from Earth?

Carbon dioxide is infrared-active. Why are not nitrogen and oxygen?
​

Why is carbon dioxide infrared-active?

I can describe to you why. But it is beyond me to explain why, in any sensible way.

The essence is related to the dipole moment of molecules of substances.


​
In principle ...
​
All molecules vibrate: They rapidly stretch and bend. [In fact, vibration of molecules is actually movement of the atoms of the molecule.]

During vibrations of the molecules of some substances, their dipole moment fluctuates: it changes rapidly, in sync with the vibrations. Such molecules can be “excited” by absorption of infrared radiation. They are said to be “infrared-active”.
 
The molecules of some other substances are such that their dipole moment does not change during the vibrations. These cannot be “excited” by absorption of infrared radiation. They are “infrared-inactive”.


Okay, that's the rule, but we need to make sense of it specifically in relation to molecules of nitrogen, oxygen, and carbon dioxide ......


​
Dipole moments of molecules in their "rest positions"
​
An important question, with huge implications for our daily lives is whether water molecules, for example, have a dipole moment - that is whether they are polar molecules.

We usually discuss this question by reference to drawings of water molecules in which the atoms appear to be stationary. In fact, the atoms are always jiggling about their positions - and that is what constitutes the vibrations of the molecule. In an accurate drawing, the apparently stationary atoms are in their average position over an extended period of time.

Picture
Portrayals of molecules of water, nitrogen, oxygen, and carbon dioxide, with their atoms in the average locations. Image courtesy of The King’s Centre for Visualization in Science at https://explainingclimatechange.com/lesson3/3_3_4.html)
We can deduce from the diagrams above that molecules of nitrogen, oxygen, and carbon dioxide do not have a dipole moment: that is, they are non-polar molecules. On the other hand, water molecules do have a dipole moment.
​

However, what is important with respect to whether a molecule is infrared-active or not, is not whether the "at rest" molecule has a dipole moment, but whether the dipole moment fluctuates during vibrations of the molecules. Two different things!

Let's see ......


​
​
Nitrogen and oxygen molecules (and other homonuclear diatomic molecules)
​

The only possible vibration of homonuclear diatomic molecules is stretching: the atoms moving farther apart and then closer together, in rapid repetitive motion. This can be regarded as stretching of the covalent bonds that hold the two atoms together as a molecule.

Picture
A representation of an oxygen atom. An animation of stretching vibrations is well portrayed at the website below.
To see animation of an oxygen molecule vibrating, click here.
If you pause the animation at any time, you can see that, regardless of how the bond is stretched, the molecule has zero dipole moment.

So, the dipole moment does not change during vibrations: it it always zero.

And so, oxygen molecules, as well as nitrogen molecules for the same reason, are not "infrared active". They do not absorb infrared radiation.

And so, they are not greenhouse gases.

​
​
And carbon dioxide molecules?
​

Molecules of carbon dioxide, with atoms in their average locations, are non-polar. So how can they be infrared-active?

Well, remember that the criterion is not about whether the molecules have a dipole moment or not, but whether the dipole moment fluctuates in sync with the molecules vibrations.

​
Carbon dioxide molecules can have four different (and simultaneous) ways, called "modes" of vibration:
  • Symmetric stretching
  • Anti-symmetric stretching
  • Bending (or scissoring) - two identical modes, except for the plane of vibration.

These are well portrayed in animations that can be viewed in a video accessible HERE.

Picture
A frame capture of the animations of the vibrational modes of carbon dioxide molecules.
Watch the animations of each mode of vibration, pausing at critical frames. You will see:
  • Symmetric stretching: The dipole moment does not change. This mode is not infrared-active. It contributes nothing to carbon dioxide’s absorption of infrared radiation, and action as a greenhouse gas.
  • Antisymmetric stretching: The dipole moment does change during vibrations. This mode is infrared-active: it can be “excited” to a higher energy level by absorption of infrared photons.
  • Bending: The dipole moment does change during vibrations. These two modes are infrared-active.

So, carbon dioxide is infrared-active, and is a greenhouse gas.
 

The absorption spectrum of carbon dioxide (for academic interest)
​
The size of the gap between the "allowed" levels of energy in the two bending modes are equal, but are smaller than the gap between the energy levels of the antisymmetric mode of vibration.

So they absorb photons of different energy (radiations of different wavelength) for energy jumps to the higher "allowed" energy level.

This can be seen in the infrared absorption spectrum of carbon dioxide ....

​
Picture
Two strong absorption peaks are due separately to (i) excitation of the asymmetric stretching mode, at 2349 wavenumbers, and (ii) excitation of both bending modes, at 667 wavenumbers. Wavenumber is a unit used by spectroscopists. The higher the wavenumber, the shorter the wavelength of radiation (and the larger the photon energy).
With acknowledgement of the excellent videoclip at https://www.youtube.com/watch?v=K6dSM_nDee8

Picture
LEARNING CHEMISTRY FOR UNDERSTANDING
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  • HOME
  • NAVIGATION
    • Table of contents
    • Index
    • TALK WITH PROF BOB?
  • LEARNING MODULES
    • Chapter 02 Stuff, matter: What is it? >
      • 0200 Stuff, matter: A theory of atoms
      • 0201 Atoms: The building blocks of all stuff
      • 0202 People classifying stuffs. Why?
    • Chapter 05 Chemical reactions, chemical equations >
      • 0500 Chemical reactions vs. chemical equations. Overview
      • 0501 Chemical amount and its unit of measurement, mole
      • 0502 The Avogadro constant: How many is that?
      • 0503 The Avogadro constant: Why is it that number?
      • 0504 Chemical formulas: What can they tell us??
      • 0505 Chemical equations: What can they tell us?
      • 0506 Limiting reactants: How much reaction can happen?
      • 0507 Balanced chemical equations: What are they?
      • 0508 Chemical reactions as competitions
    • Chapter 09 Aqueous solutions >
      • 0901 What is a solution? And what is not?
      • 0902 Miscibility of liquids in each other
      • 0903 Like dissolves like? Shades of grey
      • 0905 Dissolution of ionic salts in water: A competition
      • 0906 Can we predict solubilities of salts?
      • 0907 Solution concentration
      • 0908 Chemical species, speciation in aqueous solution
      • 0909 Solutes: Electrolytes or non-electrolytes?
      • 0910 Electrolytes - strong or weak?
      • 0911 Concentrated, dilute, strong, weak
      • 0912 Species concentration vs. solution concentration
      • 0913 Weak electrolytes: Getting quantitative
    • Chapter 11: Dynamic chemical equilibrium >
      • 1100 Dynamic chemical equilibrium: Overview
      • 1101 Visualising dynamic chemical equilibrium
      • 1102 The jargon of chemical equilibrium
      • 1103 Equilibrium constants: The law of equilibrium
      • 1104 The law of equilibrium: an analogy
    • Chapter 22 Spectroscopy >
      • 2200 Spectroscopy: Overview and preview
      • 2201 Quantisation of forms of energy
      • 2202 Light: Wave-particle "duality"
      • 2203 Ultraviolet-visible spectroscopy
      • 2204 Beer’s law: How much light is transmitted?
    • Chapter 27 The greenhouse effect, climate change >
      • 2700 The greenhouse effect: overview
      • 2701 Is Earth in energy balance?
      • 2702 CO2 in the atmosphere before 1800
      • 2703 So little CO2! Pffft?
      • 2704 Does CO2 affect Earth's energy balance?
      • 2705 The "greenhouse effect"
      • 2706 Why does CO2 absorb radiation from Earth?
      • 2707 The "enhanced greenhouse effect"
      • 2708 Why doesn't CO2 absorb the radiation from the sun?
      • 2709 Why are N2 and O2 not greenhouse gases?
      • 2710 Doesn't water vapour absorb all the IR?
      • 2711 Carbon dioxide from our cars
      • 2712 The source of energy from combustion
      • 2713 Comparing fuels as energy sources
      • 2714 Methane: How does it compare as a GHG?
      • 2715 Different sorts of pollution of the atmosphere
      • 2716 "Acidification" of seawater
    • Chapter 27 Communicating chemistry >
      • 2700 Overview, preview
      • 2703 The jargon we use
  • TEACHERS' CORNER
    • T01 Communicating chemistry
    • T02 Beer's law
    • T03 Professional amnesia of the chemistry teaching professio
    • T04 Law of equilibrium
    • T05 Visusalizing dynamic chemical equilibrium
    • Information vs. knowledge
  • PERSONAL GALLERY
    • Family
    • Travel
    • Playful dolphins
    • The University of Western Australia
    • Kings Park
    • Perth
    • At work
    • 999 Thermodynamics