Structure of the Atom

Introduction

Atomic structure is a story of evidence refining models. The guide moves from Rutherford scattering and nuclear notation to spectra, photons, nuclear size, and the Bohr model at HL.

Guide Focus

• Explain the Geiger-Marsden-Rutherford experiment and nuclear notation.
• Use spectra as evidence for discrete atomic energy levels.
• Apply HL nuclear radius, distance of closest approach, and Bohr-model ideas.

Key Concepts

1. Rutherford scattering

Most alpha particles passed through thin gold foil, but a few scattered through large angles. This showed that atoms contain a tiny, dense, positively charged nucleus.

2. Nuclear notation

A nucleus is described by chemical symbol X, proton number Z, and nucleon number A. A gives protons plus neutrons; Z gives protons.

3. Spectra and photons

Emission and absorption spectra provide evidence for discrete atomic energy levels. A photon is emitted or absorbed when an electron changes energy level, with photon energy E = hf.

4. HL nuclear and Bohr models

Nuclear radius follows R = R0 A^(1/3), implying approximately constant nuclear density. The Bohr model for hydrogen uses En = -13.6/n^2 eV and quantized angular momentum mvr = nh / (2pi).

Common Mistakes

• Confusing nucleon number with neutron number.
• Treating spectra as continuous rather than discrete line patterns.
• Forgetting that higher n levels in hydrogen are less negative and closer to zero energy.

Exam Tips

• For photon transitions, use energy difference, not absolute level alone.
• Rutherford distance of closest approach uses energy conservation.
• Spectral lines can identify chemical composition because each element has a characteristic pattern.

Practice Questions

Question 1 (Multiple Choice)

Line emission spectra provide evidence that atomic energy levels are:

A. Discrete. B. Continuous. C. Randomly changing. D. All identical for every element.

Question 2 (Structured Paper 2 Style)

An electron in hydrogen moves from n = 3 to n = 2. The levels are E3 = -1.51 eV and E2 = -3.40 eV.

(a) Calculate the photon energy in eV. [2 marks]

(b) State whether the photon is emitted or absorbed. [1 mark]