It was Bohr who incorporated the concept of discrete orbits to which electrons were confined. He based this on quantum principles that were new at the time but which have become established as scientific realities. According to the Bohr model, electrons occupy discrete orbits, and when they move to another orbit, they emit or absorb not in continuous amounts, but in bundles of energy, called quanta. Incorporating the work of Bohr and Chadwick, the modern picture of the atom looks like this: Most of the atom is empty space.
Negatively charged electrons orbit a small but heavy nucleus composed of protons and neutrons. Because quantum theory, which is based on the uncertainty principle, regards electrons as both waves and particles, they can't be definitively located. You can only talk about the likelihood of an electron being in a particular position, so the electrons form a probability cloud around the nucleus. The number of neutrons in the nucleus is usually the same as the number of protons, but it can be different.
Atoms of an element that have a different number of neutrons are called isotopes of that element. Most elements have one or more isotope, and some have several. Tin, for example, has 10 stable isotopes and at least twice as many unstable ones, giving it an average atomic mass significantly different than twice its atomic number.
If James Chadwick's discovery of the neutron had never occurred, it would be impossible to explain the existence of isotopes. Chadwick's discovery of the neutron led directly to the development of the atomic bomb. Because neutrons have no charge, they can penetrate more deeply into the nuclei of target atoms than protons. Neutron bombardment of atomic nuclei became an important method to gain information about the characteristics of nuclei.
It didn't take scientists long to discover, however, that bombarding super-heavy Uranium with neutrons was a way to break the nuclei apart and release an enormous amount of energy.
The fission of uranium produces more high-energy neutrons that break apart other uranium atoms, and the result is an uncontrollable chain reaction. Once this was known, it was only a matter of developing a way to initiate the fission reaction on demand in a deliverable casing. Fat Man and Little Boy, the bombs that destroyed Hiroshima and Nagasaki, were the result of the secret war effort known as the Manhattan Project that was conducted to do just that.
The Chadwick Atomic Theory also makes it possible to understand radioactivity. Some naturally occurring minerals — as well as manmade ones — spontaneously emit radiation, and the reason has to do with the relative number of protons and neutrons in the nucleus. A nucleus is most stable when it has an equal number, and it becomes unstable when it has more of one than another.
Chadwick went on to work on other projects, but kept thinking about the problem. Around , several researchers, including German physicist Walter Bothe and his student Becker had begun bombarding beryllium with alpha particles from a polonium source and studying the radiation emitted by the beryllium as a result. Some scientists thought this highly penetrating radiation emitted by the beryllium consisted of high energy photons. Chadwick had noticed some odd features of this radiation, and began to think it might instead consist of neutral particles such as those Rutherford had proposed.
They found that this radiation knocked loose protons from hydrogen atoms in that target, and those protons recoiled with very high velocity. In , he tried similar experiments himself, and became convinced that the radiation ejected by the beryllium was in fact a neutral particle about the mass of a proton. He also tried other targets in addition to the paraffin wax, including helium, nitrogen, and lithium, which helped him determine that the mass of the new particle was just slightly more than the mass of the proton.
Chadwick also noted that because the neutrons had no charge, they penetrated much further into a target than protons would. By it had been established that the newly discovered neutron was in fact a new fundamental particle, not a proton and an electron bound together as Rutherford had originally suggested. Scientists soon realized that the newly discovered neutron, as an uncharged but fairly massive particle, could be used to probe other nuclei.
Chadwick, whose discovery of the neutron had paved the way for the atomic bomb, worked on the Manhattan Project during WWII. He died in APS News Archives.
Librarians Authors Referees Media Students. This work is important because of how it contradicts his predecessor, Rutherford's, conclusion of what a neutron was. It was deemed in fact a new particle, rather than the combination of an electron and proton as Rutherford had once suggested.
In the last decade of his career, he served as Master of Gonville and Caius College for a decade, before retiring in Chadwick's discovery of the neutron first and foremost changed the way scientists did their work. It changed their inherent views on the atom and led to many more substantial projects, such as, the atomic bomb itself.
Chadwick got to study and learn under some of the most notable people in the history of scientific discovery: H. Mosely, Charles Darwin, Kasimir Fajans, and others just to name a few. These people were in fact influential in his own discoveries. He passed peacefully in his sleep in The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines.
The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only.
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