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Theme E — Nuclear and Quantum Physics

E.1 Structure of the Atom — IB Physics SL Revision Notes & Topical Questions

5 min read 9 parts Lesson 2 of 30 1 quiz

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Lesson summary

E.1 Structure of the Atom — IB PHYSICS SL notes: Describe the nuclear model of the atom and the evidence for it, and explain emission and absorption spectra using discrete energy levels and photons.Experiment — Geiger and Marsden, under Rutherford, fired alpha particles at thin g…

  • Observations
  • Conclusions
  • Nuclide notation
  • Proton number Z
  • Neutron number
  • Discrete energies

Part 1 of 7Alpha-particle scattering

Alpha-particle scattering
Experiment
  • Geiger and Marsden, under Rutherford, fired alpha particles at thin gold foil
Observations
  • Observations most passed straight through, a few were deflected through large angles and a very few came straight back
Conclusions
  • Conclusions the atom is mostly empty space with a small, massive, positively charged nucleus

Part 2 of 7The nucleus

Contents
  • Contents protons and neutrons, together called nucleons
Nuclide notation
  • AZX, where A is the nucleon number and Z the proton number
Proton number Z
  • Proton number Z it fixes the element
Neutron number
  • N = A − Z
Isotopes
  • Isotopes same Z but different N

Part 3 of 7Energy levels

Discrete energies
  • Discrete energies electrons in atoms can only have certain energies
Ground state
  • Ground state the lowest level, with higher ones called excited states
Negative values
  • Negative values level energies are negative because the electron is bound

Part 4 of 7Photons and transitions

Emission
  • Emission a transition from a higher to a lower level emits one photon with ΔE = hf = hc/λ
Absorption
  • Absorption it happens only if the photon energy exactly matches a level difference
Constants
  • Constants h = 6.63 × 10−34 J s and 1 eV = 1.60 × 10−19 J

Part 5 of 7Line spectra

Emission spectrum
  • Emission spectrum bright lines on a dark background from a hot low-pressure gas
Absorption spectrum
  • Absorption spectrum dark lines on a continuous spectrum when light passes through a cooler gas
Same wavelengths
  • Same wavelengths the lines occur at the same wavelengths for a given element
Use
  • Use spectra give evidence for the chemical composition of stars and gases

Part 6 of 7How a line is produced

1
An electron is excited to a higher level by heating, collision or absorbing a photon.
2
The excited state is unstable.
3
The electron drops to a lower level.
4
A photon with energy equal to the level difference is emitted.
5
Different transitions give different lines in the spectrum.

Part 7 of 7Emission and absorption compared

FeatureEmissionAbsorption
AppearanceBright lines on a dark backgroundDark lines on a continuous spectrum
Produced byHot, low-pressure gasWhite light through a cooler gas
Electron transitionHigher to lower levelLower to higher level
Line positionsCharacteristic of the elementSame wavelengths as emission

Key terms

TermMeaning
Nucleon numberThe total number of protons and neutrons in a nucleus.
Proton numberThe number of protons in a nucleus.
IsotopesAtoms of the same element with different numbers of neutrons.
Energy levelOne of the discrete energies an electron can have in an atom.
PhotonA quantum of electromagnetic energy, E = hf.
Emission spectrumThe set of discrete wavelengths emitted by excited atoms of an element.
Absorption spectrumA continuous spectrum crossed by dark lines where a gas has absorbed particular wavelengths.

Avoid these mistakes

Avoid these mistakes
Saying the nucleus fills the atom — the atom is mostly empty space.
Thinking a photon of any energy can be absorbed — only photons matching an energy gap are absorbed.
Confusing the proton number with the nucleon number.

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