Thomson [4] a year later was an indication that the atom had internal structure. At the beginning of the 20th century the accepted model of the atom was J. Thomson's 'plum pudding' model in which the atom was a positively charged ball with smaller negatively charged electrons embedded inside it. In the years that followed, radioactivity was extensively investigated, notably by Marie and Pierre Curie as well as by Ernest Rutherford and his collaborators. By the turn of the century physicists had also discovered three types of radiation emanating from atoms, which they named alpha, beta, and gamma radiation.
Experiments by Otto Hahn in and by James Chadwick in discovered that the beta decay spectrum was continuous rather than discrete. That is, electrons were ejected from the atom with a continuous range of energies, rather than the discrete amounts of energy that were observed in gamma and alpha decays.
This was a problem for nuclear physics at the time, because it seemed to indicate that energy was not conserved in these decays. The Nobel Prize in Physics was awarded jointly to Becquerel for his discovery and to Marie and Pierre Curie for their subsequent research into radioactivity. Rutherford was awarded the Nobel Prize in Chemistry in for his 'investigations into the disintegration of the elements and the chemistry of radioactive substances'.
In Albert Einstein formulated the idea of mass—energy equivalence. While the work on radioactivity by Becquerel and Marie Curie predates this, an explanation of the source of the energy of radioactivity would have to wait for the discovery that the nucleus itself was composed of smaller constituents, the nucleons. More work was published in by Geiger and Ernest Marsden, [7] and further greatly expanded work was published in by Geiger. The key experiment behind this announcement was performed in at the University of Manchester: Ernest Rutherford's team performed a remarkable experiment in which Geiger and Marsden under Rutherford's supervision fired alpha particles helium nuclei at a thin film of gold foil.
The plum pudding model had predicted that the alpha particles should come out of the foil with their trajectories being at most slightly bent.
But Rutherford instructed his team to look for something that shocked him to observe: a few particles were scattered through large angles, even completely backwards in some cases. He likened it to firing a bullet at tissue paper and having it bounce off. The discovery, with Rutherford's analysis of the data in , led to the Rutherford model of the atom, in which the atom had a very small, very dense nucleus containing most of its mass, and consisting of heavy positively charged particles with embedded electrons in order to balance out the charge since the neutron was unknown.
As an example, in this model which is not the modern one nitrogen consisted of a nucleus with 14 protons and 7 electrons 21 total particles and the nucleus was surrounded by 7 more orbiting electrons. Around , Arthur Eddington anticipated the discovery and mechanism of nuclear fusion processes in stars, in his paper The Internal Constitution of the Stars. This was a particularly remarkable development since at that time fusion and thermonuclear energy, and even that stars are largely composed of hydrogen see metallicity , had not yet been discovered.
The Rutherford model worked quite well until studies of nuclear spin were carried out by Franco Rasetti at the California Institute of Technology in Rasetti discovered, however, that nitrogen had a spin of 1.
With the discovery of the neutron, scientists could at last calculate what fraction of binding energy each nucleus had, by comparing the nuclear mass with that of the protons and neutrons which composed it. Differences between nuclear masses were calculated in this way. Alexandru Proca was the first to develop and report the massive vector bosonfield equations and a theory of the mesonic field of nuclear forces.
In Hideki Yukawa [18] proposed the first significant theory of the strong force to explain how the nucleus holds together. In the Yukawa interaction a virtual particle, later called a meson, mediated a force between all nucleons, including protons and neutrons. This force explained why nuclei did not disintegrate under the influence of proton repulsion, and it also gave an explanation of why the attractive strong force had a more limited range than the electromagnetic repulsion between protons.
Later, the discovery of the pi meson showed it to have the properties of Yukawa's particle. With Yukawa's papers, the modern model of the atom was complete. The center of the atom contains a tight ball of neutrons and protons, which is held together by the strong nuclear force, unless it is too large.
Unstable nuclei may undergo alpha decay, in which they emit an energetic helium nucleus, or beta decay, in which they eject an electron or positron. After one of these decays the resultant nucleus may be left in an excited state, and in this case it decays to its ground state by emitting high energy photons gamma decay.
The study of the strong and weak nuclear forces the latter explained by Enrico Fermi via Fermi's interaction in led physicists to collide nuclei and electrons at ever higher energies. This research became the science of particle physics, the crown jewel of which is the standard model of particle physics which describes the strong, weak, and electromagnetic forces.
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Software Images icon An illustration of two photographs. Images Donate icon An illustration of a heart shape Donate Ellipses icon An illustration of text ellipses. Modern Physics, 3rd Edition Item Preview. EMBED for wordpress. End-of-chapter problem sets now include problems organized according to chapter section, which offer the student an opportunity to gain familiarity with a particular topic, as well as general problems, which often require the student to apply a broader array of concepts or techniques.
The range of abilities required to solve the problems has been broadened, so that this edition includes both more straightforward problems that build confidence as well as more difficult problems that will challenge students. Each chapter now includes a brief summary of the important points. Some of the end-of-chapter problems are available for assignment using the WebAssign program www.
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