AP Chemistry Unit 3 Study Notes

AP Chemistry 3.5: Photons and the Electromagnetic Spectrum

Relate photon energy, wavelength, frequency, and electronic transitions.

Aligned to Properties of Substances and Mixtures from the current College Board AP Chemistry course outline. Exam weighting for this unit: 18%-22% of the multiple-choice score range listed by College Board.

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Spectroscopy and the Electromagnetic Spectrum

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These detailed Unit 3 notes were organized from the provided study document. For further study, visit Khan Academy. All Khan Academy content is available for free at www.khanacademy.org.

Spectroscopy and the Electromagnetic Spectrum Open

  • Spectroscopy studies how matter interacts with electromagnetic radiation.

    • Different forms of electromagnetic radiation differ in wavelength, frequency, and photon energy .

  • The electromagnetic spectrum includes, from longer wavelength/lower frequency toward shorter wavelength/higher frequency:

    • radio → microwave → infrared → visible → ultraviolet → X-ray → gamma

  • Wavelength is represented by λ .

    • Frequency is represented by ν .

    • The relationship is:

    • c = λν

    • where c is the speed of light.

    • Because c is constant in vacuum, wavelength and frequency are inversely related:

    • λ ↑ → ν ↓

    • and:

    • λ ↓ → ν ↑

  • Photon energy is:

    • E = hν

    • where h is Planck's constant.

    • Therefore higher frequency means higher photon energy:

    • ν ↑ → E ↑

    • Combining the relationships means:

    • shorter wavelength → higher frequency → higher energy

    • and:

    • longer wavelength → lower frequency → lower energy

  • A useful combined equation is:

    • E = hc/λ

    • This lets you calculate photon energy from wavelength.

    • Common constants:

    • c ≈ 3.00 × 10⁸ m/s

    • h ≈ 6.626 × 10⁻³⁴ J·s

  • Be careful with wavelength units.

    • If λ is given in nm, convert to meters when using c in m/s:

    • 1 nm = 1 × 10⁻⁹ m

  • Different parts of the electromagnetic spectrum interact with matter in different ways.

    • Microwave radiation can be associated with molecular rotational changes.

    • Infrared radiation commonly interacts with molecular vibrations.

    • Visible and ultraviolet radiation can cause electronic transitions in suitable substances.

    • Higher-energy radiation such as X-rays can interact with more tightly bound electrons.

    • Khan Academy's current Unit 3 contains an introduction to spectroscopy and the electromagnetic spectrum.

  • An absorption spectrum shows wavelengths/frequencies of radiation absorbed by a sample.

  • An emission spectrum shows radiation emitted by excited particles.

  • Spectra can be used to identify substances because allowed energy levels are characteristic of particular atoms or molecules.