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3.11Properties of Substances and Mixtures

Spectroscopy and the Electromagnetic Spectrum — AP CHEMISTRY Revision Notes & Topical Questions

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Lesson summary

3.11 Spectroscopy and the Electromagnetic Spectrum — Notes — AP CHEMISTRY notes: Relate regions of the electromagnetic spectrum to molecular and electronic transitions.From low to high energy the regions are radio, microwave, infrared, visible, ultraviolet, X-ray and gamma.Shorte…

  • Absorption spectrum
  • Emission spectrum
  • Rotational transition
  • Vibrational transition
  • Shorter wavelength means higher frequency and higher photon energy
  • Microwave radiation changes molecular rotational energy levels

Section 1 of 8The spectrum

The spectrum
From low to high energy the regions are radio, microwave, infrared, visible, ultraviolet, X-ray and gamma.
Shorter wavelength means higher frequency and higher photon energy.

Section 2 of 8Effect of each region

1
Microwave radiation changes molecular rotational energy levels.
2
Infrared radiation changes vibrational energy levels (bond stretching and bending), and IR spectra identify functional groups.
3
Ultraviolet and visible radiation cause electronic transitions, promoting valence electrons to higher energy levels.
4
X-rays can eject core electrons, as in photoelectron spectroscopy.
RegionWavelengthEnergyEffect on moleculeUse
MicrowavecmLowRotationMicrowave ovens
InfraredμmMediumVibrationIR spectroscopy
Visible400-700 nmHigherElectron transitionsColour
UltravioletBelow 400 nmHighElectron transitions and bond breakingUV-visible spectroscopy

Section 3 of 8Quantised levels

Quantised levels
Energy levels in atoms and molecules are quantised.
Only photons whose energy exactly equals the gap between levels are absorbed.

Section 4 of 8Absorption and emission

Absorption and emission
An absorption spectrum shows which wavelengths are absorbed.
An emission spectrum shows the wavelengths emitted as excited electrons fall to lower levels.
Each element has a unique line spectrum.

Section 5 of 8Colour

Colour
The colour of a solution is the complementary colour of the light it absorbs.
A solution that absorbs orange light looks blue.

Section 6 of 8More from the specification

More from the specification
Quantized electronic energy shells
  • Quantized electronic energy shells absorbing incident photons promotes valence electrons between distinct atomic energy shells,
    • producing characteristic dark absorption lines.
Molar absorptivity
  • How strongly a species absorbs light at a wavelength.
Absorbance
  • Measure of light absorbed by a sample.
Concentration
  • Moles ÷ volume in dm³, or g/dm³.
Calibration
  • Checking an instrument against a known standard and adjusting it.
Shell
  • An energy level around the nucleus that holds a limited number of electrons.
Calibration curve
  • A graph of absorbance against known concentrations.

Section 7 of 8Key terms

TermMeaning
Electromagnetic spectrumRange of radiation from radio waves to gamma rays.
QuantisedRestricted to discrete energy values.
Absorption spectrumSpectrum of wavelengths absorbed by a sample.
Emission spectrumSpectrum of wavelengths emitted by excited species.
Rotational transitionChange in molecular rotation caused by microwaves.
Vibrational transitionChange in bond vibration caused by infrared radiation.

Section 8 of 8Exam technique

1
Saying longer wavelength means higher energy — the relationship is inverse
2
Mixing up the effects — microwave causes rotation, infrared vibration and UV-visible electronic changes
3
Saying any photon can be absorbed — its energy must match an energy gap
4
Thinking the colour seen is the colour absorbed — it is the complement

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