Short on time? This page is the recap. For explanations, diagrams and worked examples, use the full study guide.
Recap
- Electromagnetic radiation delivers energy in packets called photons.
- In order of increasing photon energy (decreasing wavelength): radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.
- A molecule stores energy in rotation, vibration and electron arrangement. Each is quantized.
- The gaps between levels grow in the order rotational ≪ vibrational ≪ electronic.
- A photon is absorbed only if its energy matches a gap. So the region a substance absorbs in tells you the type of transition.
- Emission is the reverse: the species drops down a gap and gives out a photon of matching energy.
Key relationships
| Region | Approximate wavelength | Transition | What changes |
|---|---|---|---|
| Microwave | 1 mm to 1 m | rotational | how fast the molecule spins |
| Infrared | 700 nm to 1 mm | vibrational | how strongly bonds stretch and bend |
| Visible | 400 nm to 700 nm | electronic | an electron moves to a higher level |
| Ultraviolet | 10 nm to 400 nm | electronic | an electron moves up; bonds may break |
Colour: a substance looks roughly the complementary colour of the light it absorbs (absorbs orange → looks blue).
Assumptions behind the model
- Energy levels are quantized and the three kinds of gap are well separated in size.
- Region boundaries are approximate; near an edge, the region only roughly predicts the transition.
- Single atoms have no bonds, so they cannot vibrate and show no molecular rotational spectrum; they show only electronic transitions.
Mistakes to avoid
- Reversing the energy order. Long wavelength means low energy, not high.
- “Infrared excites electrons.” No: infrared is vibrational.
- “Microwaves cause vibration.” No: microwaves are rotational.
- Comparing wavelengths in different units. Convert mm, μm and nm to metres first.
- Naming the absorbed colour as the colour seen. You see what is left.
- Confusing brightness with photon energy. More photons do not make each photon more energetic.
Quick self-check
- Which has more energy per photon, infrared or ultraviolet? (Ultraviolet)
- A gas absorbs at 3.5 μm. What type of transition is this? (Vibrational: 3.5 μm is infrared)
- Why does neon gas show no infrared absorption? (Single atoms have no bonds to vibrate)
- A solution absorbs violet light. Roughly what colour does it look? (Yellow)
Next: practice questions.