Climate Change Investigation 2-3: Why Do Some Molecules
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![]() Carbon dioxide gas allows visible light to pass through but will absorb infrared energy. Why is that? The answer involves a concept called resonance. If you are pushing a child on a swing, in order to get the child to swing as high as possible, you have to time your pushes just right. If the swing's natural cycle (frequency) without pushing is one swing every second, you must push at that frequency—once per second. You must also give the push at just the right instant, when the swing is just about to go forward. That "just right" pushing frequency needed to transfer energy from one system (you) to another (the swinger) is called the resonant frequency. A fascinating aspect of resonant frequency is that you can feel it. When you’re pushing someone on a swing, you know when you have found the resonant frequency because you can feel your energy being transferred to the swing and the person on it. Each time you push, you can see the swing go higher and higher and higher.
Materials Although there are a many combinations of materials that can be used to construct model molecules for this investigation, the following work well, are inexpensive, and readily available:
* Cable ties available at hardware stores. To represent flexible molecular bonds, it may also be possible to use chenille stems (pipe cleaner), strips of springy plastic, thin flexible rods, stiff wire, or springs. Wooden materials such as Tinker toys or toothpicks are not flexible enough, so don’t use them. ** If you use chenille stems, twist two of them evenly around each other four times.
Pinch the ends securely so they don’t unwind). You will have 7 "bonds" when you
are finished which you will cut to 12 or 15 cm lengths for your models.
The 12 cm lengths represent strong bonds and the 15 cm lengths
represent weak bonds. [Note: there are two illustrative videos using chenille stems in the GSS Teacher Guide for Climate Change Chapter 2 .]
What To Do
1. Construct your models following the instructions below:
2. Test each model to determine its resonant frequency. For example, hold the carbon dioxide molecule by the central carbon atom and shake it up and down a few inches. Try a range of shaking speeds, or frequencies, from very slow (one shake per second) to very fast (seven or eight shakes per second). QUESTIONS
2.3.1 Which of the four molecules has the fastest resonant frequency? 2.3.2 If there are differences in resonant frequencies, why do you think they are different? 2.3.3 The behavior of these models are analogies to the behavior of real molecules of carbon dioxide, methane, oxygen, and nitrogen. From the observations of your models and their interactions with different frequencies of vibration, why do some gases in the atmosphere absorb infrared radiation while others don’t? Disclaimer: The models in this investigation are adequate to explore the basic concept of a resonant frequency, but a weakness in the model is that real bonds behave differently from plastic cable ties or pipe cleaners. The oxygen double bond model using two ties and the nitrogen triple bond model using three ties are much stiffer than they should be to represent real bonds. Moreover carbon dioxide has double bonds between the carbon atom and the two oxygen atoms, but in our models they are represented by single cable ties to make it easier for the molecule models to vibrate. Additional insight can be seen in this 3-minute animation from MinuteEarth: How Do Greenhouse Gases Actually Work? This investigation is very similar to one found on the NOAA Earth System Research Laboratory, Education and Outreach site. Specifically, Earth System Interactions, Teaching Activity: What's So Special About CO2? [Teaching Guide and Student Handouts.] Other activities on resonance may be found at
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