3.9 Spectroscopy and the Electromagnetic Spectrum

The electromagnetic spectrum

Electromagnetic radiation travels as waves with wavelength (λ), frequency (ν), and energy related by c = λν and E = hν. The spectrum ranges from low-energy, long-wavelength radio waves to high-energy, short-wavelength gamma rays, passing through microwave, infrared, visible, ultraviolet, and X-ray regions. Energy and frequency increase together; wavelength decreases as energy increases.

Molecular interactions with light

Different energies of radiation excite different molecular motions. Microwave radiation causes molecular rotations; infrared (IR) radiation causes bond vibrations (stretching/bending), useful for identifying functional groups; UV/visible light excites electronic transitions, moving electrons between energy levels. Absorption occurs when a photon's energy exactly matches the gap between two states.

Spectroscopy as an analytical tool

Spectroscopy measures how matter absorbs or emits electromagnetic radiation to identify substances and determine structure. IR spectroscopy reveals bond types via characteristic absorption frequencies. UV-Vis spectroscopy relates to electronic structure and, via Beer's Law (A = εbc), allows determination of concentration in solution from measured absorbance.

Key terms

c = λν

Speed of light equals wavelength times frequency; used to relate wavelength and frequency of EM radiation.

E = hν

Energy of a photon equals Planck's constant times frequency; higher frequency means higher energy.

Infrared (IR) spectroscopy

Uses IR radiation to excite bond vibrations, helping identify functional groups in molecules.

UV-Vis spectroscopy

Uses UV/visible light to probe electronic transitions; used with Beer's Law to find concentration.

Beer's Law

A = εbc; absorbance is directly proportional to concentration, molar absorptivity, and path length.

Try an AP-style question

Which type of radiation causes electronic transitions in atoms and molecules?

1. Infrared
2. Ultraviolet/visible
3. Microwave
4. Radio