Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

Theme E — Nuclear and Quantum Physics

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

5 min read 12 parts Lesson 2 of 45 1 quiz

📚 IB Physics HL Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
0%read
Lesson summary

E.1 Structure of the Atom — IB PHYSICS HL notes: Describe the alpha-scattering evidence for the nucleus, nuclear notation, atomic spectra, photon energy, nuclear radius and density, deviations from Rutherford scattering and the Bohr model.Apparatus — an alpha source in a lead con…

  • Lead container
  • Thin gold foil
  • Evacuated chamber
  • Observation A
  • Observation B
  • Observation C

Part 1 of 9Nuclear notation

Nuclear notation
Subatomic particles
  • Subatomic particles protons (+1.60 × 10−19 C, 1.673 × 10−27 kg), neutrons (0, 1.675 × 10−27 kg) and electrons (−1.60 × 10−19 C, 9.109 × 10−31 kg)
Atomic mass unit
  • Atomic mass unit masses are also given in u, with proton 1.007276 u, neutron 1.008665 u and electron 0.000549 u
Notation
  • AZX
Nucleon number A
  • Nucleon number A the total number of protons and neutrons
Proton number Z
  • Proton number Z the number of protons, which fixes the element
Neutron number
  • N = A − Z
Isotopes
  • Isotopes the same Z but different N

Part 2 of 9Emission and absorption spectra

Origin
  • Origin atoms emit or absorb light of certain wavelengths because electrons make transitions between discrete energy levels
Emission spectrum
  • Emission spectrum heating a low-pressure gas excites electrons
  • De-excitation emits photons, giving coloured lines on a black background
Absorption spectrum
  • Absorption spectrum white light through a cool low-pressure gas loses photons that exactly match transitions
  • The result is dark lines on a coloured background at the same wavelengths as the emission lines
Fingerprint
  • Fingerprint every element has a unique set of lines, so composition can be found
Examples
  • Examples hydrogen has a strong red line at 656 nm, sodium a yellow line at 589 nm and mercury lines mostly below 450 nm

Part 3 of 9Photons and energy levels

Photon
  • Photon a massless packet or quantum of electromagnetic energy
  • It transfers its energy all at once, unlike a wave
Energy levels
  • Energy levels electrons occupy the lowest available level (ground state)
  • Level energies are negative because the electron is bound
Excitation
  • Excitation absorbing a photon raises the electron to a higher level
De-excitation
  • De-excitation emitting a photon lowers it
Ionisation
  • Ionisation removing the electron from the atom
  • Ionisation energy is the minimum energy to remove an electron from the ground state
Energy level diagram
  • Energy level diagram horizontal lines with the ground state lowest and the ionisation level at 0 at the top
Number of lines
  • Number of lines n levels give n(n − 1)/2 possible transitions

Part 4 of 9Photon energy

Equations
  • E = hf = hc/λ
    What the symbols mean: E = energy of one photon (J) · h = Planck constant (6.63 × 10−34 J s) · f = frequency (Hz)
Level difference
  • Level difference hf = E2 − E1 and λ = hc/(E2 − E1)
Trend
  • Trend a larger energy difference gives a higher frequency and shorter wavelength
Constants
  • Constants h = 6.63 × 10−34 J s and 1 eV = 1.60 × 10−19 J
Photon rate
  • Photon rate the number of photons per second is the power divided by the energy of one photon

Part 5 of 9Nuclear radius and density (HL)

Nuclear radius
  • R = R0A1/3 with the Fermi radius R0 = 1.20 × 10−15 m
Graph
  • R against A1/3 is a straight line through the origin with gradient R0
Volume
  • V = (4/3)πR3 ∝ A, so each nucleon occupies about the same volume
Nuclear density
  • ρ = 3u/(4πR03), the same for all nuclei and independent of radius
Value
  • Value about 2 × 1017 kg m−3
  • It shows the nucleus holds most of the atom's mass and the atom is mostly empty space

Part 6 of 9Closest approach (HL)

Method
  • Method an alpha particle fired head-on stops momentarily where all its kinetic energy has become electric potential energy
Equation
  • Ek = k(2e)(Ze)/d so d = 2kZe2/Ek
Charges
  • Charges the alpha particle has charge 2e and the nucleus Ze
Meaning
  • Meaning d is an upper limit for the nuclear radius provided only electrostatic force acts
Scattering law
  • Scattering law the number scattered through angle θ is proportional to 1/sin4(θ/2)

Part 7 of 9Deviations from Rutherford scattering (HL)

Prediction
  • Prediction the number scattered falls as the angle increases
Agreement
  • Agreement correct at low to moderately high energies
High energies
  • High energies above about 27.5 MeV the number back-scattered falls sharply to zero
Reason
  • Reason the alpha particles get close enough to feel the strong nuclear force
    • (below about 1.5 fm)
Evidence
  • Evidence this shows the strong force exists and gives the size of the nucleus
Largest deviation
  • Largest deviation high-energy alpha particles and targets of low nucleon number such as aluminium

Part 8 of 9Bohr model of hydrogen (HL)

Line series
  • Lyman, Balmer (to n = 2, visible) and Paschen (to n = 3, infrared)
    • (to n = 1, ultraviolet)
Orbits
  • Orbits electrons move only in fixed orbits of certain radii
Energy levels
  • En = −13.6/n2 eV
Angular momentum
  • Angular momentum mvr = nh/2π
Limit
  • Limit it explains hydrogen but fails for atoms with more than one electron

Part 9 of 9More from the specification

More from the specification
Rutherford Alpha Particle Scattering Experiment
  • Directing alpha particles at a thin gold foil showed that while most traversed undeflected, a small fraction scattered at large angles, proving that atomic mass and positive charge are concentrated in a tiny central nucleus.
Quantised Transitions and Photon Emission
  • When an atomic electron transitions between discrete bound energy states, the emitted photon energy equals the exact energy difference: delta_E = hf = hc / lambda.
Discrete Spectral Lines at Specific Wavelengths
  • Because atomic electron energy levels are quantised, each element exhibits a characteristic emission spectrum consisting of sharp lines at specific wavelengths.
Electrostatic Turning Point and Closest Approach
  • An alpha particle approaching head-on converts all its initial kinetic energy into electrostatic potential energy at minimum distance r0,
  • where Ek = * (q1 q2 / r0).
    • (1 / 4 pi epsilon0)

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 spectrumBright lines on a dark background from a hot gas.
Absorption spectrumDark lines on a continuous spectrum from light passing through a cool gas.
Closest approachThe distance at which an alpha particle stops momentarily as its kinetic energy becomes potential energy.

Avoid these mistakes

1
Saying the atom is mostly solid because most alpha particles bounced back — most passed straight through.
2
Confusing nucleon number and proton number.
3
Using the closest approach distance as the exact nuclear radius — it is an upper limit.
4
Taking R proportional to A instead of A1/3.
5
Forgetting to convert eV or MeV to joules in photon calculations.

Last updated:

\n