AP® Chemistry review sheet from Aim for Five (aimforfive.com/chem/units/3/3-11)
Unit 3 · Topic 3.11
3.11 Spectroscopy and the Electromagnetic Spectrum
Different regions of the electromagnetic spectrum cause different changes in molecules. Microwaves change how molecules rotate, infrared radiation changes how bonds vibrate, and ultraviolet and visible light move electrons to higher energy levels.
Key terms
- electromagnetic spectrum
- microwave radiation
- infrared radiation
- ultraviolet and visible light
- molecular vibration
- electronic transition
The electromagnetic spectrum
Light is electromagnetic radiation, and visible light is a tiny part of a much wider range. Going from low energy to high energy, the main regions are radio waves, microwaves, infrared (IR), visible, ultraviolet (UV), X-rays and gamma rays. Higher energy goes with higher frequency and shorter wavelength (topic 3.12).
Within visible light, red has the longest wavelength (about 700 nm) and lowest energy, and violet has the shortest (about 400 nm) and highest energy.
Approximate wavelength ranges
The boundaries between regions are fuzzy, so you won't need exact values. What matters is the order: as wavelength increases from UV toward microwaves, the energy of each photon decreases. Rough ranges:
| Region | Approximate wavelength range |
|---|---|
| Ultraviolet | about 10 nm to 400 nm |
| Visible | about 400 nm (violet) to 700 nm (red) |
| Infrared | about 700 nm to 1 mm |
| Microwave | about 1 mm to 1 m |
What each region does to molecules
Rotational, vibrational and electronic energy levels are all quantized, meaning only certain amounts of energy are allowed. A molecule absorbs a photon only if the photon's energy matches the gap between two of its levels. The table summarizes which region matches which kind of change.
| Region | Relative energy | What it changes in a molecule |
|---|---|---|
| Microwave | low | rotational energy (how fast the molecule spins) |
| Infrared | medium | vibrational energy (bonds stretching and bending) |
| Visible and ultraviolet | high | electronic energy (an electron moves to a higher energy level) |
Why this is useful
Each region tells chemists something different. Infrared spectra show which kinds of bonds a molecule has, because different bonds vibrate at different frequencies. That's how an IR spectrum can show whether a molecule contains O–H or C=O bonds. UV-visible spectra show electronic transitions, which is why colored solutions absorb visible light and why spectrophotometers (topic 3.13) work.
The same idea explains everyday effects. CO₂ and H₂O molecules in the atmosphere absorb infrared radiation given off by Earth's surface, because IR matches the energy of their bond vibrations. Sunscreen molecules absorb UV light by moving electrons to higher energy levels.
An absorption spectrum is a graph of absorbance against wavelength. Peaks show the wavelengths a substance absorbs strongly. Chemists use the peak wavelength to identify a substance and choose a wavelength for measuring its concentration.
Absorption and emission
When a molecule absorbs a photon, it moves to a higher energy state. When it drops back down, it can emit a photon. In both cases, the photon's energy equals the energy difference between the two states. Topic 3.12 adds the equations for that.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Matching a region to a transition
A gas sample is exposed to three kinds of radiation: microwaves, infrared and ultraviolet. Which one could promote an electron to a higher energy level? Which one would mainly make its bonds vibrate more?
Show the solutionHide the solution
- Step 1: Moving an electron to a higher energy level takes the most energy of the three changes. Only UV (or visible) light has enough energy per photon.
- Step 2: Bond vibrations need less energy, which matches infrared photons.
- Step 3: Microwaves are lowest in energy and change rotational states.
Answer: Ultraviolet promotes electrons; infrared increases bond vibrations.
- Example 2
More light isn't more energy per photon (classic trap)
A student shines a very bright infrared lamp on a molecule, expecting it to cause electronic transitions, since the lamp delivers a lot of energy. Explain why that doesn't happen.
Show the solutionHide the solution
- Step 1: Absorption happens one photon at a time. Each photon's energy must match the gap between energy levels.
- Step 2: A brighter lamp sends more photons, but each infrared photon still carries the same, fairly small amount of energy.
- Step 3: IR photons don't have enough energy for an electronic transition, so they're absorbed only by vibrational transitions.
Answer: Brightness changes the number of photons, not the energy of each; individual IR photons are too low in energy to cause electronic transitions.
Common mistakes
- Mixing up the regions: microwave = rotation, infrared = vibration, UV/visible = electronic.
- Thinking longer wavelength means higher energy. It's the reverse.
- Thinking brighter light means higher-energy photons. Brightness is the number of photons.
- Ordering the regions incorrectly, such as putting UV below visible in energy.
On the exam
- Expect short questions asking which type of radiation causes a given change in a molecule, or why a substance absorbs in a particular region. Name both the region and the type of transition.
- This topic often pairs with photon calculations (3.12) or spectrophotometry (3.13) in a longer question.
Connected topics
Videos
Check yourself
4 questions on 3.11 Spectroscopy and the Electromagnetic Spectrum. Pick an answer to see if you got it, and why.
A chemist uses infrared spectroscopy to study a sample of CH₃OH. Which of the following processes occurs when the molecules absorb infrared radiation?
A gas-phase molecule absorbs radiation at three different wavelengths: 2 cm, 5 μm and 250 nm. Which of the following correctly matches each wavelength with the change it causes in the molecule?
A solution of a dye looks orange because the dye absorbs blue light. What happens in the dye molecules when they absorb this light?
Which of the following lists types of electromagnetic radiation in order of increasing photon energy?
0 of 4 answered