Chapter 9 · Electrons in Atoms and the Periodic Table
9.3The Electromagnetic Spectrum
9 min · two checks
Predict
Where does visible light sit relative to ultraviolet and infrared?
The idea
- Order the main regions of the spectrum by energy.
- Connect photon energy to a chemical or biological effect.
From low energy to high: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma ray. Visible light runs roughly from 750 nm (red) to 400 nm (violet). The order of colors is the order of increasing energy toward violet. Ultraviolet photons can break bonds in skin and in some molecules. X-rays and gamma rays are ionizing. Infrared is what you feel as radiant heat; its photons make molecular motions more vigorous without the damage of ionizing radiation.
Atoms and molecules absorb only certain photon energies, the ones that match a jump between allowed states. That is why a gas excited by a discharge emits a line spectrum — a barcode of colors — rather than a rainbow. Those lines are evidence that electron energy is quantized. Continuous rainbows come from hot dense solids, not from isolated atoms.
Keep these
- Energy order: radio < microwave < IR < visible < UV < X-ray < gamma.
- Visible light is about 400–750 nm.
- Line spectra show that atomic energies are quantized.