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Section 4: Inside the Nucleus—Protons and Neutrons
Rutherford's gold foil experiment showed that the atom has a dense area of positive charge at the center, but it would take further investigations to discover exactly what was inside the nucleus. Unlike the previous discoveries of the electron and the nucleus, these new particles were theorized first, then gradually confirmed through a series of experiments.
Figure 3-6. English Chemist William Prout
William Prout was born in Gloucestershire, England, and practiced medicine in London. At the same time, he found time to conduct experiments in chemistry and compared the densities of different elements in their gaseous form at the same temperature and pressure. He discovered that the density of most of the elements he tested were multiples of the density of hydrogen, from which he inferred that the atomic weights of the heavier elements must also be multiples of the atomic weights of hydrogen as well. He is remembered today mainly for what is called Prout's hypothesis.
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Near the beginning of the 19th century, the English chemist William Prout (1785–1850) pointed out that the atomic masses of all the elements seemed to be multiples of a particular fundamental particle, which he called the "protyle." The protyle, which has the same mass as hydrogen, was declared to be the building block of all elements. While more accurate measurements of atomic mass seemed to cast doubt on this hypothesis, Prout's theory was an influence on Rutherford, who, in 1917, set up a series of experiments to see if he could separate atoms into smaller components. Rutherford's chosen technique was to knock particles out of different gases by bombarding them with alpha particles.
Rutherford initially set up a source of alpha particles to target a chamber filled with hydrogen. He and his team observed that the collisions produced a new particle that traveled much farther than the original alpha particles—leading to the conclusion that it had a smaller mass than the alpha particle. These same particles resulted when they filled the chamber with other light elements such as nitrogen, fluorine, and phosphorous. Since the new particle, which was found to have a positive charge as well, had an identical signature on their detectors as a hydrogen nucleus, they hypothesized that this particle must be the same as a hydrogen atom's nucleus. Since this particle was knocked out of the nuclei of all of the elements on which they tried this experiment, Rutherford and his colleagues concluded that every atom must contain these positive particles in the nucleus.
In 1919, Rutherford officially announced this discovery—a new subatomic particle with a mass 1,800 times greater than an electron's and a positive charge that was the same size but opposite sign to the electron's charge. He named it the proton, a nod perhaps to Prout's protyle. The proton, one of which is found in the hydrogen atom, is found in multiple numbers in all the other elements. We recognize the proton today as one of the building blocks of the atomic nucleus and the basis of the positive charge found in the nucleus of all atoms.
Between Rutherford's discoveries of the nucleus and the proton, scientists began to notice clear patterns in how big the positive charge was on the nucleus. Antonius van den Broek (1870–1926) was an amateur physicist who was the first to notice this. He said each element had a different charge on its nucleus, and then in later experiments by Henry Moseley (1887–1915), it was confirmed. Moseley called this unique number Z, the atomic number for an element. With Rutherford's discovery of the proton, it became clear that this atomic number was the number of protons in the nucleus. (Moseley will be covered in more depth in Unit 4.)
Figure 3-7. The Atom That Chadwick Proposed with the Neutron
Following his work in Rutherford's lab at Cambridge, British physicist James Chadwick won the 1935 Nobel Prize in Physics for his 1932 discovery of the neutron.
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The neutron was the last major subatomic particle to be discovered. Experiments showed that, except for hydrogen, there were fewer protons in the nucleus than in the atomic mass. This means that, assuming a proton has a mass of one, each element, if it had only protons in the nucleus, would have a mass that matched its atomic number. Helium, for example, has two protons, but an atomic mass of four. Rutherford, in fact, predicted the existence of the neutron fairly soon after discovering the proton, but he postulated that it was simply a proton and an electron paired up inside the nucleus whose net electrical charge added up to zero.
In the decades after World War I, scientists continued to experiment with different types of radiation. In 1931, Walther Bothe (1891–1957) and his student, Herbert Becker, in Germany discovered that when alpha particles bombarded light elements, such as beryllium, lithium, or boron, an unusually penetrating radiation was produced. James Chadwick (1891–1974), a former student of Ernest Rutherford, investigated this new type of radiation and determined that it consisted of a beam of neutral particles, which have a high penetrating power because they are not repelled by the nucleus as are alpha particles. These three particles—electrons, protons, and neutrons—together serve as the subatomic building blocks of the atom.
To summarize: The electron is light and negatively charged, and exists in a region outside the nucleus. Most of the mass of an atom consists of the protons and neutrons—each one of which is approximately 1,800 times more massive than an electron. This places most of the mass, and all the positive charge, inside the nucleus. In a neutral atom, the numbers of protons and electrons is equal, so the total charge balances out. Finally, within the nucleus, the number of neutrons can be anywhere from zero to almost double the number of protons, though there are almost always more neutrons than protons in the nucleus. As we shall see in the next unit, for many elements, the number of neutrons can vary, even for one specific number of protons. These variants of the different elements are called "isotopes."
The discovery of the neutron was an important milestone in the long investigations leading to the modern concept of the atom, but it was by no means enough on its own to complete the story. For that, we have to look again at the electron and its interactions with light to bring into focus the rest of the picture: the quantum view of the atom.