Geiger-Marsden's Gold Foil Experiment & Rutherford's Model of the Atom

This topic is part of the HSC Physics course under the section Structure of The Atom. 

HSC Physics Syllabus

  • investigate, assess and model the experimental evidence supporting the nuclear model of the atom, including:

– the Geiger-Marsden experiment
– Rutherford’s atomic model
– Chadwick’s discovery of the neutron (ACSPH026)

  • assess the limitations of the Rutherford and Bohr atomic models

Geiger-Marsden's Gold Foil Experiment

Geiger and Marsden performed an experiment using a thin gold foil to investigate the structure of the atom.

 

Rutherford’s Model of the Atom

Rutherford's model of the atom is characterised by a few key features:

  • a highly concentrated positively charged region in the centre of the atom, called the nucleus
  • most of the atom is empty space
  • electrons orbit the nucleus

Geiger-Marsden's Gold Foil Experiment

Geiger and Marsden fired alpha particles (helium nuclei) at a thin gold foil. The gold foil was surrounded by a screen that would cause scintillations when alpha particles hit it. 

 

What Did the Gold Foil Experiment Show?

  • Observation 1: Most alpha particles passed through the gold foil undeflected as most scintillations were observed directly behind the gold foil. 

This observation supported Rutherford's postulate that an atom is mostly empty space. 

 

  • Observation 2: A few alpha particles were deflected and some of which were reflected back (large angle deflections). The angle of deflection was measured by the position at which they were detected on the fluorescent screen.

This observation supported the presence of a region in the atom of highly concentrated positive charge (nucleus). In an atom of gold, the charge and mass of the nucleus are substantially greater than that of an alpha particles. As a result, when an alpha particle collided with the nucleus, it was reflected.

 

gold foil experiment and atomic models

 

Geiger and Marsden's gold foil experiment not only provided evidence for Rutherford's model of the atom, it rejected the preceding atomic model proposed by Thomson. Although Thomson's model predicted that all, if not most, alpha particles would pass through the gold foil undeflected, it could not account for the few alpha particles that were reflected. 

How was the Proton Discovered?

After Geiger and Marsden's gold foil experiment, Rutherford tried to investigate the content of the nucleus. 

Rutherford fired alpha particles at a sample of nitrogen gas, which resulted in a transmutation reaction producing protons. 

 

alpha particle and nitrogen transmutation

 

Rutherford conducted a similar experiment as Thomson to determine the value of the charge to mass ratio of a proton. He showed that a proton is positively charged and much heavier than an electron. 

Although Rutherford demonstrated that the charge of proton(s) accounts for the positive nature of the nucleus, he wasn't able to account for the nuclear mass.

Limitations of Rutherford's Model of the Atom

There are three main limitations to Rutherford's atomic model:

1. The model fails to explain the stability of electrons' orbital motion.

Rutherford proposed that electrons must be orbiting the positively charged nucleus like how satellites orbit the Earth, otherwise they would clash into the nucleus due to the electrostatic attraction towards the nucleus. However, by Maxwell's electromagnetic theory, electrons should emit radiation when they experience centripetal acceleration. By the law of conservation of energy, an electron's kinetic energy should gradually decrease, leading to its spiralling motion into the nucleus. 

     

    In classical physics, accelerating particles emit energy including ones undergoing circular motion. In this case, why do electrons not emit energy despite experiencing electrostatic attraction towards a positive nucleus? 
     
    2. The model does not describe the motion nor location of electrons. 
    Rutherford's model only described electrons to be orbiting the nucleus. He did not describe the exact orbital radius, nor the orbital velocity of electrons.
     
    3. The model does not describe the constituent of the nucleus.
    While Rutherford later discovered protons which accounted for the positive charge of the nucleus, his model and scientific knowledge at the time were unable to account for the atomic mass of elements. This was only later explained by the discovery of the neutron by Chadwick. 

     

    Previous section: Millikan's Oil Drop Experiment

    Next section: Chadwick's Discovery of The Neutron

     

    BACK TO MODULE 8: FROM THE UNIVERSE TO THE ATOM