AP® Chemistry · Unit 3 · Topic 3.11
Spectroscopy and the Electromagnetic Spectrum
Light comes in photons whose energy depends only on frequency, not intensity, and each energy range matches a different kind of molecular motion: rotation, vibration, or electron jumps.
Every photon carries energy given by E = h x nu, where h is Planck's constant and nu is frequency. High-frequency photons carry more energy each; low-frequency photons carry less. A molecule can only absorb a photon if that photon's energy matches a gap between two of the molecule's allowed energy states. This is why the spectrum is divided into regions that each talk to a different kind of molecular motion.
Microwave photons are low energy and match the spacing between rotational energy levels, so microwaves make molecules like water spin faster, which we feel as heat. Infrared photons carry more energy and match vibrational levels, the stretching and bending of bonds. Ultraviolet and visible photons carry even more energy and can match the gaps between electronic energy levels, kicking an electron into a higher orbital or knocking it loose entirely. A lamp's visible light mostly bounces off your food without matching any gap that would heat it efficiently, which is why a microwave oven, tuned to water's rotational transitions, heats food that a lamp does not.
This sets up the next idea: a beam's intensity only controls how many photons arrive per second, not how much energy each one carries. Piling on more red photons never substitutes for one higher-energy UV photon, because the requirement is energy per photon, not total energy delivered. If a photon's energy does not match any gap in the molecule, nothing happens: the photon passes through or scatters, unabsorbed. This idea of matching photon energy to allowed gaps is also the basis for spectroscopy, where absorbed wavelengths reveal a substance's identity and concentration.
Key terms
- Photon
- A discrete packet of light energy whose size depends only on its frequency.
- Planck's constant (h)
- The proportionality constant, 6.626 x 10−34 J s, linking a photon's frequency to its energy.
- Rotational energy levels
- Quantized spinning states of a molecule, excited by low-energy microwave photons.
- Vibrational energy levels
- Quantized bond stretching and bending states, excited by infrared photons.
- Electronic energy levels
- Quantized states describing electron positions in a molecule, excited by visible or ultraviolet photons.
Watch it explained
- How it worksHow does each region of the spectrum match a different kind of molecular motion?
- How it worksCompare what microwave, infrared, and ultraviolet photons each do to a molecule they hit.
- Common mistakeWhy isn't a more intense beam of red light able to do what one ultraviolet photon does?
- Going furtherWhat happens to a molecule when the photon energy does not match any of its energy gaps?
Check yourself
End of topic.