17.1 - Early Models of the Atom and the Discovery of the Nucleus
Objectives:
- To know the history of the discovery of the atom.
- To be able to describe the Geiger-Marsden experiment (Gold Foil) and its results.
J. J. Thompson - discovered the electron.
Democritus was a pre-Socratic Greek materialist philosopher (born at Abdera in Thrace ca. 460 BCE - died ca 370 BCE). Democritus was a student of Leucippus and co-originator of the belief that all matter is made up of various imperishable, indivisible elements which he called atoma (sg. atomon) or "indivisible units", from which we get the English word atom. It is virtually impossible to tell which of these ideas were unique to Democritus and which are attributable to Leucippus.
The model of the atom in the days of Newton was a tiny, hard, indestructible sphere. Although this model worked well for the kinetic theory of gases (macroscopic behaviour); new models of the atom had to be devised when later experiments revealed the electrical nature of atoms.
J. J. Thomson suggested a model of the atom as a volume of positive charge with negative electrons (discovered in 1895 by Thomson) embedded through the atom. This model adopted the name of ‘plum pudding’ due its resemblance of the tasty sweet (very English) dish.
Ernest Rutherford (1871 – 1937) was a physicist working at the University of Manchester. In 1911 he had two enthusiastic students called Geiger and Marsden. To keep them busy he asked them to experiment with a piece of gold foil. He suggested that they fire alpha particles at the foil and watch them bounce off the gold atoms. Alpha particles were not known to be anything else other than very heavy, very dense charged particles. He did not expect them to find anything interesting, he thought that the alpha particles would just be scattered when they passed the large ‘plum pudding’ atoms.
A steady source of alpha particles is channelled into a thin beam by a lead shield and aimed at a thin piece of gold foil. Although a piece of paper could be used to create a thin beam of alpha particles the lead shield was used to stop natural background gamma rays passing into the apparatus. As the alpha particles pass through the foil they are scattered before impacting onto a glass shield. The glass is coated on the inside with a layer of a metallic sulphide. When a particle hits the coating a small flash of light (scintillation) is produced. The light emitted can be detected using a microscope. As it was known that alpha particles were absorbed by the air in only a few cm the experiment was performed in a vacuum.
Experimental Observations
The results were startling and finished Thomson’s model of the atom. The majority of the alpha particles passed straight through the gold foil as if there was nothing there with some being scattered by small amounts. However an occasional particle was scattered through more than 90 degrees (in practice this number was less than 1 in 8000). While this was a tiny amount it was statistically a very large number of particles. Geiger and Marsden had to count many thousands of flashes before they could analyse their data.
No one could believe that a massive alpha particle could bounce off a gold atom. Rutherford wrote ‘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’.
Rutherford was able to show that the large angle scattering was due to a single encounter between an alpha particle and an intense electric charge. He concluded that:
An atom has a densely charged core (nucleus) which contains the vast majority of the mass of the atom and which is surrounded by orbiting electrons.
Consequently the nucleus occupies a very small proportion of the available space. Contrary to many texts Rutherford did not know that the atom had a positive charge, at this time the charge of an alpha particle was unknown. He did use the term ‘nucleus’, but only knew that it had the same charge as an alpha particle.
This classical model of the atom as one of a dense positive nucleus surrounded by electrons in orbit is still taught in schools today and serves as an excellent foundation to build upon. Unfortunately as soon as the model was published concerns arose.
The main problem came from the orbits of electrons around a nucleus. Scientists imagined them to be similar to planets orbiting a star, with the exception that the atom is governed by the electromagnetic force rather than the gravitational force. These electrons were undergoing circular motion; as such they must be acted upon by a centripetal force. Unfortunately it was known that if a charge (in this case an electron) is accelerated it will emit electromagnetic radiation (light). As the electron radiates (loses) energy its radius of orbit should steadily decrease and its frequency of revolution will increase. This leads to a constantly increasing amount of radiation and a collapse of the atom as the electron plunges downwards into the nucleus. Rutherford’s classical model of the atom was finished and soon the entire world of classical physics was to come crumbling down.
The model of the atom in the days of Newton was a tiny, hard, indestructible sphere. Although this model worked well for the kinetic theory of gases (macroscopic behaviour); new models of the atom had to be devised when later experiments revealed the electrical nature of atoms.
J. J. Thomson suggested a model of the atom as a volume of positive charge with negative electrons (discovered in 1895 by Thomson) embedded through the atom. This model adopted the name of ‘plum pudding’ due its resemblance of the tasty sweet (very English) dish.
Ernest Rutherford (1871 – 1937) was a physicist working at the University of Manchester. In 1911 he had two enthusiastic students called Geiger and Marsden. To keep them busy he asked them to experiment with a piece of gold foil. He suggested that they fire alpha particles at the foil and watch them bounce off the gold atoms. Alpha particles were not known to be anything else other than very heavy, very dense charged particles. He did not expect them to find anything interesting, he thought that the alpha particles would just be scattered when they passed the large ‘plum pudding’ atoms.
A steady source of alpha particles is channelled into a thin beam by a lead shield and aimed at a thin piece of gold foil. Although a piece of paper could be used to create a thin beam of alpha particles the lead shield was used to stop natural background gamma rays passing into the apparatus. As the alpha particles pass through the foil they are scattered before impacting onto a glass shield. The glass is coated on the inside with a layer of a metallic sulphide. When a particle hits the coating a small flash of light (scintillation) is produced. The light emitted can be detected using a microscope. As it was known that alpha particles were absorbed by the air in only a few cm the experiment was performed in a vacuum.
Experimental Observations
The results were startling and finished Thomson’s model of the atom. The majority of the alpha particles passed straight through the gold foil as if there was nothing there with some being scattered by small amounts. However an occasional particle was scattered through more than 90 degrees (in practice this number was less than 1 in 8000). While this was a tiny amount it was statistically a very large number of particles. Geiger and Marsden had to count many thousands of flashes before they could analyse their data.
No one could believe that a massive alpha particle could bounce off a gold atom. Rutherford wrote ‘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’.
Rutherford was able to show that the large angle scattering was due to a single encounter between an alpha particle and an intense electric charge. He concluded that:
An atom has a densely charged core (nucleus) which contains the vast majority of the mass of the atom and which is surrounded by orbiting electrons.
Consequently the nucleus occupies a very small proportion of the available space. Contrary to many texts Rutherford did not know that the atom had a positive charge, at this time the charge of an alpha particle was unknown. He did use the term ‘nucleus’, but only knew that it had the same charge as an alpha particle.
This classical model of the atom as one of a dense positive nucleus surrounded by electrons in orbit is still taught in schools today and serves as an excellent foundation to build upon. Unfortunately as soon as the model was published concerns arose.
The main problem came from the orbits of electrons around a nucleus. Scientists imagined them to be similar to planets orbiting a star, with the exception that the atom is governed by the electromagnetic force rather than the gravitational force. These electrons were undergoing circular motion; as such they must be acted upon by a centripetal force. Unfortunately it was known that if a charge (in this case an electron) is accelerated it will emit electromagnetic radiation (light). As the electron radiates (loses) energy its radius of orbit should steadily decrease and its frequency of revolution will increase. This leads to a constantly increasing amount of radiation and a collapse of the atom as the electron plunges downwards into the nucleus. Rutherford’s classical model of the atom was finished and soon the entire world of classical physics was to come crumbling down.
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A modern version of the famous Gold Foil Experiment
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17.2 - Spectra and Electron Orbits (Bohr Model)
Objectives:
- To know how spectral lines are seen.
- To know that that spectral lines are caused by the emission and absorption of light of specific frequencies.
- To understand that the spectral lines are due to electrons gaining or losing energy as they jump from one energy level to another.
- To be able to calculate the photon frequency from a given series of jumps.
17.3 - The Photoelectric Effect
Objectives:
- To be able to describe the observations from the photoelectric effect and why it was so important to the story of the atom.
- To be able to explain the reason why there exists a threshold frequency and be able to interpret a graph of the observational data.
- To be able to carry out the simulation of the photoelectric effect and calculate the work function and Planck's constant from a set of data.
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17.4 - Wave-Particle Duality
Objectives:
- Be able to describe the wave effects of electrons (Young's Slit experiment).
- Be able to calculate the De Broglie wavelength of the wave associated with a given particle.
- Be able to calculate the effective momentum of a photon given its wavelength or frequency.
- To be able to describe how the wave-like behaviour of electrons explains the quantum nature of electron orbits and their associated energy levels.
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17.5 - Electron Scattering
Objectives:
- To able to calculate the frequency change when a photon hits an electron using the conservation of momentum.
- To be able to calculate the photon wavelengths from electron-positron annihilation.
17.6 - Radioactivity and Nuclear Decay
Objectives:
- To recall the three decay modes of the nucleus and the reason why.
- Be able to complete nuclear reaction equations.
- To be able to calculate the half-life of an isotope given a set of data.
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17.7 - Energy and Mass (Einstein's Equation!)
Objectives:
- To know that matter can be converted into energy and vice versa.
- To be able to calculate the energy released in several examples: nuclear fission and positron-electron annihilation.
- To know and use the famous equation: \(E = mc^{2} \)
