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3.12Properties of Substances and Mixtures

Properties of Photons — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 7 parts Lesson 19 of 20 1 quiz

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3.12 Properties of Photons — Notes — AP CHEMISTRY notes: Use the photon equations and relate photon energy to electronic transitions.Light behaves as a stream of photons, packets of electromagnetic energy.Light shows wave-particle duality, and the photoelectric effect showed its …

  • Wave equation
  • Planck's equation
  • What the symbols mean
  • Energy per mole of photons
  • Convert nanometres to metres first, where 1 nm
  • When an electron moves between levels, ΔE

Section 1 of 7Photons

Photons
Light behaves as a stream of photons, packets of electromagnetic energy.
Light shows wave-particle duality, and the photoelectric effect showed its particle nature.
When an atom or molecule absorbs or emits a photon, its energy changes by the energy of the photon.

Section 2 of 7Equations

1Wave equation
  • Wave equation c = λν, with c = 2.998 × 108 m s−1, λ in metres and ν in s−1 (Hz).
2Planck's equation
  • E = hν, where h = 6.626 × 10−34 J s.
3Combined
  • E = hc/λ.
    What the symbols mean: E = photon energy (J) · h = 6.63 × 10−34 J s · c = 3.00 × 108 m s−1 · λ = wavelength (m)
4Energy per mole of photons
E(photon) × NA, with NA = 6.022 × 1023 mol−1.
5Convert nanometres to metres first, where 1 nm
1 × 10−9 m.

Section 3 of 7Electronic transitions

When an electron moves between levels, ΔE
Eupper − Elower = hν.
Absorption
  • Absorption the electron moves to a higher level and energy is taken in.
Emission
  • Emission the electron falls to a lower level and a photon is released.
Larger energy gaps give photons of higher frequency and shorter wavelength.
Quantised levels give line spectra of specific wavelengths, not a continuous band.

Section 4 of 7Hydrogen spectrum

Bohr showed electrons exist only in fixed energy shells.
Transitions to n
1 lie in the ultraviolet, to n = 2 in the visible and to n = 3 in the infrared.
GivenFindFormula
λνν = c/λ
νEE = hν
λEE = hc/λ
E per photonE per moleMultiply by NA

Section 5 of 7More from the specification

More from the specification
Photoelectric emission from excited atoms
  • Photoelectric emission from excited atoms photons transferring quantum energy hf to surface atoms eject bound electrons when energy exceeds work functions.
Calibration curve
  • A graph of absorbance against known concentrations.
Infrared radiation
  • Long-wavelength radiation emitted by warm surfaces.
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.

Section 6 of 7Key terms

TermMeaning
PhotonPacket of electromagnetic energy.
Planck's constant6.626 × 10−34 J s relating energy and frequency.
FrequencyNumber of wave cycles per second.
WavelengthDistance between consecutive wave crests.
Line spectrumSpectrum of discrete wavelengths from quantised transitions.
Photoelectric effectEmission of electrons from a metal when light shines on it.

Section 7 of 7Exam technique

1
Using wavelength in nanometres without converting to metres
2
Saying energy and wavelength are directly proportional — they are inversely related
3
Forgetting to multiply by Avogadro's number for energy per mole
4
Describing line spectra as continuous

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