E.1
Structure of the Atom
Rutherford's scattering, atomic structure, emission and absorption spectra, energy levels and photon emission. HL extends to nuclear radius, density and the Bohr model.
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Key Concepts, Structure of the Atom
Rutherford's Scattering Experiment
Geiger and Marsden fired alpha particles at a thin gold foil and observed where they went. Three key observations led to Rutherford's nuclear model. Most alpha particles passed straight through (the atom is mostly empty space). Some were deflected through small angles. A very small number bounced back through angles greater than 90°. Rutherford concluded that almost all the mass of the atom is concentrated in a tiny, dense, positively charged nucleus. The large deflections result from the electrostatic repulsion between the positive alpha particles and the positive nucleus. This replaced the Thomson 'plum pudding' model in which charge was spread throughout the atom.
Atomic Structure and Notation
An atom consists of a tiny, dense, positively charged nucleus containing protons and neutrons (nucleons), surrounded by a cloud of electrons. The nucleus has a diameter of approximately 10⁻¹⁵ m, while the atom is about 10⁻¹⁰ m across, making the atom roughly 100,000 times larger than its nucleus. The atomic number Z is the number of protons and defines the element. The mass number A is the total number of nucleons. Standard notation places A at the top left and Z at the bottom left of the chemical symbol. A neutral atom has Z electrons.
Emission and Absorption Spectra
When a gas is excited (by heat or electrical discharge), electrons jump to higher energy levels and then fall back, emitting photons of specific frequencies. The result is a line emission spectrum: bright coloured lines on a dark background. When white light passes through a cool gas, electrons absorb photons of those same specific frequencies, producing a line absorption spectrum: a continuous rainbow with dark lines at exactly the same positions. Each element has a unique set of spectral lines, acting as a fingerprint. This is how astronomers identify the chemical composition of stars.
Energy Levels and Photon Emission
Electrons in atoms exist only in discrete (quantised) energy levels. When an electron drops from a higher level E₂ to a lower level E₁, it releases a photon with energy E = E₂ - E₁ = hf, where h = 6.63 × 10⁻³⁴ J s is Planck's constant and f is the frequency. Since c = fλ, the photon wavelength is λ = hc/(E₂ - E₁). Energy levels are usually given in electronvolts (eV), where 1 eV = 1.6 × 10⁻¹⁹ J. The ground state is the lowest energy level. Only specific frequencies of light are emitted or absorbed, producing line spectra rather than continuous spectra.
Nuclear Radius and Density (HL)
From high-energy scattering experiments, nuclear radius follows R = R₀A^(1/3), where R₀ ≈ 1.2 × 10⁻¹⁵ m and A is the mass number. Nuclear volume is proportional to A, and since mass is also proportional to A, all nuclei have approximately the same density (around 10¹⁷ kg/m³), vastly greater than ordinary matter. The distance of closest approach in a head-on alpha scattering experiment is found using energy conservation: all kinetic energy of the alpha particle converts to electrostatic potential energy at the turning point, giving d = kZe × 2e / E_k. When higher energy alpha particles are used, they penetrate deeper and the scattering results deviate from predictions, indicating the strong nuclear force is being encountered.
The Bohr Model (HL)
Bohr proposed that electrons orbit the nucleus only in specific circular orbits where angular momentum is quantised: mvr = nh/2π, where n is a positive integer (the principal quantum number). An electron in a given orbit has a fixed energy and does not radiate. Energy is only emitted or absorbed when an electron transitions between orbits. For hydrogen, the energy of level n is E_n = -13.6/n² eV, where n = 1 is the ground state. The Bohr model correctly predicts the hydrogen spectrum but fails for more complex atoms; it is a stepping stone to full quantum mechanics.
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Frequently Asked Questions, IB Physics Structure of the Atom
What is Structure of the Atom in IB Physics? ↓
Rutherford's scattering, atomic structure, emission and absorption spectra, energy levels and photon emission. HL extends to nuclear radius, density and the Bohr model. This topic is part of Theme E (Nuclear & Quantum Physics) in the current IB Physics syllabus.
Is Structure of the Atom SL or HL in IB Physics? ↓
Structure of the Atom is covered by both SL and HL students in the current IB Physics syllabus. HL students study additional depth and extension content beyond the SL core.
What equations do I need for IB Physics Structure of the Atom? ↓
The key equations for Structure of the Atom are covered in the concept tutorial above. For a structured set of notes with all equations, conditions and worked examples, the GradePod Exam Pack includes a revision note template for every topic.
What are common exam mistakes in IB Physics Structure of the Atom? ↓
Common mistakes are covered in detail in the exam technique video above. The GradePod Exam Pack also includes exam-style questions with mark schemes so you can see exactly how marks are awarded and where students typically drop them.
How do I revise Structure of the Atom for the IB Physics exam? ↓
Follow the GradePod three-step method. First, watch the concept tutorial and tick off each learning objective on the checklist above as you go. Second, watch the exam technique video to see how IB-style questions are answered under exam conditions. Third, use the Exam Pack to practise independently with knowledge questions, exam questions and mark schemes. That's it. It works. I promise.