Nuclear Model of the Atom – Learn
The Geiger–Marsden Experiment
J.J Thomson’s investigation of cathode rays led him to develop the ‘plum pudding’ model of the atom which consisted of very small negatively charged electrons spread throughout a positive charge of very low density.
Under the guidance of Ernest Rutherford, Geiger and Marsden were directed to investigate the scattering of alpha particles by firing them at a thin foil made of gold. In trying to confirm Thomson’s model of the atom, Rutherford expected all of the alpha particles to pass through undeflected, or at most, very small angles. Marsden and Geiger found that most of the alpha particles passed through undeflected but to the surprise of Rutherford, they found that about 1/8000 deflected at angles greater than 90°.
Rutherford knew that the small negatively charged electrons would not create a large enough electrostatic force to deflect the large positive alpha particle as observed. He concluded that the charge throughout the atom was not uniform at all and that the atom must consist of a small, dense, positively charged nucleus surrounded by smaller negatively charged electrons. This led Rutherford to develop his model of the atom.
Rutherford’s Atomic Model
Ernest Rutherford had described a new model of the atom, building upon the plum pudding model which had earlier been suggested by JJ Thomson. Thomson had been successful in isolating the electron, however, the positive pudding from his model was rejected following Rutherford’s interpretation of the Gieger-Marsden experiment, as discussed previously.
Consequently, Rutherford suggested a nuclear model of the atom that obeyed the following principles:
- The vast majority of the mass of an atom occupied an extremely small and dense area in the middle of the atom. He coined this the nucleus.
- The nucleus contained all of the positive charge of the atom.
- Electrons occupied the large amounts of empty space that surrounded the nucleus of the atom.
Chadwick’s Discovery of the Neutron
After Rutherford discovered that the atom consisted of a nucleus, it was logical to assume that the nucleus consisted of protons and electrons. However, there were problems that arose as a result of this assumption. Rutherford proposed that a neutral particle similar in size to a proton must exist in the nucleus and he called this theoretical particle the neutron. Rutherford and Chadwick continued to make observations and see if any of there results suggested the existence of the neutron, but failed to confirm its existence.
In 1930, Walther Bothe and Herbert Becker fired alpha particles at beryllium and found that a highly penetrating radiation resulted. This radiation also seemed to be unaffected by electric and magnetic fields. All available evidence pointed to this radiation being a gamma ray. However, the observed radiation seemed to be more highly penetrating than previous observations of gamma rays.
Frederic Joliot and Irene Curie studied this new type of radiation by allowing it to be incident on paraffin which is highly rich in hydrogen atoms. They discovered that this radiation knocked protons off the paraffin.
Now that charged particles were involved in the observations it was much easier to make quantitative measurements of their experiments. They calculated that many more protons were emitted than should have been expected had the interaction been with gamma rays. Another problem was the energy of the emitted protons. The emitted protons had an energy equal to 5MeV. Applying conservation of momentum and energy realised an energy of 50MeV for the supposed gamma rays incident on the paraffin. This was impossible because the alpha particles in the first part of the experiment had an average energy of 5MeV – gamma rays could not create 10 times more energy!
Chadwick, who had been a student of Rutherford, knew from previous studies that gamma rays were not capable of dislodging and emitting heavy protons. Chadwick correctly interpreted the work of Bothe and Becker, Joliot and Curie as evidence for the existence of the neutron. He determined that the mass of the neutron was similar to that of the proton and subsequent experiments by Chadwick confirmed his assertion.
After confirmation and acceptance of Chadwicks results, the atomic model now comprised of a nucleus consisting of neutrons and protons: