The structure of the atom
Section: Particle Physics | Syllabus: Cambridge AS Level Physics 9702
Early Models of the Atom For thousands of years, atoms were thought to be indivisible - the smallest particles of matter. However, the discovery of cathode rays in the 1860s challenged this view. Experiments showed these rays were beams of fast-moving, negatively charged particles with mass much smaller than a hydrogen atom - now called electrons .
Since atoms are electrically neutral but contain negative electrons, physicists inferred there must be a positive component. J.J. Thomson proposed the "plum pudding" model : negative electrons were spread evenly through a diffuse (spread out) positive charge, like fruit in a plum pudding.
FIG 11.1: Thomson's Plum Pudding Model A spherical atom with diffuse positive charge (shaded sphere) containing small negative electrons (minus signs) distributed throughout, like chocolate chips in a muffin.
The diffuse positive charge and embedded electrons should be clearly labelled. The Alpha-Particle Scattering Experiment In 1908, Ernest Rutherford at the University of Manchester proposed using alpha particles (α-particles) to probe inside the atom.
Alpha particles were ideal because they are: Smaller than atoms Positively charged (+2e) Have relatively high kinetic energy Experimental Setup The experiment, carried out by Hans Geiger and Ernest Marsden, used the following apparatus: A radium source emitting α-particles A thin gold foil (about 2000 atoms thick) A fluorescent screen coated with zinc sulfide that emitted a flash of light when struck by an α-particle The apparatus was enclosed in an evacuated metal box (air removed) to prevent scattering off air molecules FIG 11.2: Rutherford's Alpha-Particle Scattering Apparatus Labelled diagram showing: radium source emitting alpha particles; lead shield confining radiation into a narrow beam; thin gold foil (about 2000 atoms thick) in the centre; circular fluorescent screen surrounding the foil.
Arrows indicate particle paths - most going straight through, some deflecting at small angles, and very few bouncing back. Observations Observation Proportion Most α-particles passed straight through undeflected Vast majority Some α-particles were deflected through small angles Small fraction Very few α-particles were deflected through angles greater than 90° (back-scattered) About 1 in 20,000 Rutherford's Reaction Rutherford described the back-scattering result: "It was quite the most incredible event that has ever happened to me in my life.
It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you." Conclusions The results could not be explained by Thomson's plum pudding model - a diffuse spread of charge could not deflect an α-particle backwards.
Observation Conclusion Most α-particles passed through undeflected The atom is mostly empty space Some α-particles deflected through small angles There is a concentration of positive charge in the atom that repels the positive α-particles Very few α-particles scattered through large angles (>90°) The positive charge and most of the mass is concentrated in a very small region - the nucleus FIG 11.3: Alpha Particle Paths Through Gold Foil Cross-section view showing gold atoms as large circles with tiny positive nuclei at their centres.
Multiple alpha particle paths shown: (1) most passing straight through empty space, (2) some deflecting at small angles near a nucleus, (3) very few bouncing back at large angles on head-on approach. Label the nucleus, electron cloud, and deflection paths.
Note: Diagram not to scale. Exam Tip When explaining the scattering experiment, always link each observation to its conclusion. Simply stating the observations without the inferences will not gain full marks.
The Nuclear Model of the Atom Rutherford proposed a model with a small, dense, positively charged nucleus surrounded by a 'cloud' of electrons. The discovery of the neutron by James Chadwick in 1932 completed the model.
Key features of the nuclear model: The atom has a small, dense, positively charged nucleus at its centre The nucleus contains protons (positive charge) and neutrons (no charge) Protons and neutrons are collectively called nucleons Electrons (negative charge) orbit the nucleus The atom is mostly empty space FIG 11.4: Nuclear Model of the Atom Central nucleus (dense cluster of protons marked + and neutrons) surrounded by orbiting electrons (marked −) at a much larger radius.
Include a key: proton (positive), neutron (neutral), electron (negative). Note: Diagram not to scale - the atom is about 10,000 times larger than the nucleus. Nucleon A particle found in the nucleus; either a proton or a neutron.
Scale of the Nucleus Rutherford used the experimental results to calculate: Diameter of the nucleus: approximately 10^-15 m Diameter of the atom: approximately 10^-10 m The atom is about 10,000 times larger than the nucleus Scale Analogy If an atom were the size of a sports stadium, the nucleus would be the size of a small marble pla…
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