The nucleus of an atom is the very dense region in its center consisting of protons and neutron. The size of the nucleus is much smaller than the size of the atom itself, and almost all of the mass in an atom is made up from the protons and neutrons with almost no contribution from the electrons.
The nucleus of an atom is made up of very tightly bound protons and neutrons. The electromagnetic force which causes like charges to repel prevents protons from binding together without neutrons (it would blow such a nucleus apart). When neutrons and protons are in very close proximity they are held together by the strong nuclear force. The strong force is much much stronger than gravity or the electromagnetic force, but because it only works over very short distances (as opposed to gravity and electromagnitism which have infinite range) we don't usually notice it in everyday life. The element hydrogen is the only element which exists whos nuclei doesn't need neutrons to hold it together, and this is because the hydrogen nucleus consists of only a single proton! The stable form of helium, the next lightest element, has two protons and two neutrons. Most of the light elements are stable when they have roughtly even numbers of protons and neutrons, but as elements get heavier and heavier they need more neutrons to stay together.
The isotope of an atom is determined by the number of neutrons in the nucleus. Different isotopes of the same element have very similar chemical properties because chemical reactions depend almost entirely on the number of electrons that an atom has. Different isotopes in a sample of a particular chemical can be separated by using a centrifuge or by using a spectrometer. The first method is used in producing enriched uranium from a sample of regular uranium, and the second is used in carbon dating.
The number of protons and neutrons together determine the nuclide (type of nucleus). Protons and neutrons have nearly equal masses, and their combined number, the mass number, is approximately equal to the atomic mass of an atom. The combined mass of the electrons is very small in comparison to the mass of the nucleus, since protons and neutrons weigh roughtly 2000 times more than electrons.
If a nucleus has too few or too many neutrons it may be unstable, and will decay after some period of time. For example, nitrogen atoms with 16 neutrons (nitrogen-16) beta decays to oxygen-16 within a few seconds of being created. In this decay a neutron in the nitrogen nucleus is turned into a proton and an electron by the weak nuclear force. The element of the atom changes because while it previously had seven protons (which makes it nitrogen) it now has eight (which makes it oxygen). Many elements have multiple isotopes which are stable for weeks, years, or even billions of years.
The radius of a nucleon (neutron or proton) is of the order of 1 fm (femtometre = 10-15 m). The nuclear radius, which can be approximated by the cubic root of the mass number times 1.2 fm, is less than 0.01 % of the radius of the atom. Thus the density of the nucleus is more than a trillion times that of the atom as a whole. One solid cubic millimetre of nuclear material, if compressed together, would have a mass of around 200,000 tonnes. Neutron stars are composed of such material.
The discovery of the electron was the first indication that the atom had internal structure. At the turn of the 20th century the accepted model of the atom was JJ Thomson's "plum pudding" model in which the atom was a large positively charged ball with small negatively charged electrons embedded inside of it. By the turn of the century physicists had also discovered three types of radiation coming from atoms, which they named alpha, beta, and gamma radiation. Experiments in 1911 by Lise Meitner and Otto Hahn, and by James Chadwick in 1914 discovered that the beta decay spectrum was continuous rather than discrete. That is, electrons were ejected from the atom with a range of energies, rather than the discrete amounts of energies that were observed in gamma and alpha decays. This was a problem for nuclear physics at the time, because it indicated that energy was not conserved in these decays.
Around the same time that this was happening (1911) Ernest Rutherford performed a remarkable experiment in which he fired alpha particles (helium nuclei) at a thin film of gold foil. The plum pudding model predicted that the alpha particles should come out of the foil with their trajectories being at most slightly bent. He was shocked to discover that many particles were scattered through large angles, even completely backwards in some cases. The discovery led to the Rutherford model of the atom, in which the atom has a very small, very dense nucleus made up of heavy positivly charged particles with embedded electrons in order to balance out the charge. As an example, in this model nitrogen-14 was made up of a nucleus with 14 protons and 7 electrons, and the nucleus was surrounded by 7 more orbiting electrons.
The Rutherford model worked quite well until studies of nuclear spin were carried out by Franco Rasetti at the California Institute of Technology in 1929. By 1925 it was known that protons and electrons had a spin of 1/2, and in the Rutherford model of nitrogen-14 the 14 protons and six of the electrons should have paired up to cancel each others spin, and the final electron should have left the nucleus with a spin of 1/2. Rasetti discovered, however, that nitrogen-14 has a spin of one.
In 1930 Wolfgang Pauli was unable to attend a meeting in Tubingen, and instead sent a famous letter with the classic introduction "Dear Radioactive Ladies and Genlemen". In his letter Pauli suggested that perhaps there was a third particle in the nucleus which he named the neutron. He suggested that it was very light (lighter than an electron), had no charge, and that it did not readily interact with matter (which is why it hadn't yet been detected). This desparate way out solved both the problem of energy conservation and the spin of nitrogen-14, the first because the neutron was carrying away the extra energy and the second because an extra neutron paired off with the elecrton in the nitrogen-14 nucleus giving it spin one. Pauli's neutron was renamed the neutrino (italian for little neutral one) by Enrico Fermi in 1931, and after about thirty years it was finally demonstrated that a neutrino really is emitted during beta decay.
In 1932 Chadwick realized that radiation that had been observed by Walther Bothe, Herbert Becker, Irene Joliot-Curie and Fredrick Joliot-Curie was actually due to a massive particle that he called the neutron. In the same year Dmitrij Iwanenko suggested that neutrons were in fact spin 1/2 particles and that the nucleus contained neutrons and that there were no electrons in it, and Francis Perrin suggested that neutrinos were not nuclear particles but were created during beta decay. To cap the year off Fermi submitted a theory of the neutrino to Nature (which the editors rejected for being "too remote from reality"). Fermi continued working on his theory and published a paper in 1934 which placed the neutrino on solid theoretical footing. In the same year Hideki Yukawa proposed the first significant theory of the strong force to explain how the nucleus holds together.
With Fermi and 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. Unstable nuclei may undergo alpha decay, in which they emit an energetic helium nuclei, 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 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 unifies the strong, weak, and electromagnetic forces.
When two light nuclei come into very close contact with each other it is possible for the strong force to fuse the two together. It takes a great deal of energy to push the nuclei close enough together for the strong force to have an effect, so the process of nuclear fusion can only take place at very high temperatures or high densities. Once the nuclei are close enough together the strong force overcomes their electromagnetic repulsion and squishes them into a new nucleus. A very large amount of energy is released when light nuclei fuse together because the binding energy per nucleon increases with atomic number up until iron. Stars like our sun are powered by the fusion of four protons into a helium nucleus, two positrons, and two neutrinos. The fusion of hydrogen into helium is also the source of energy for thermonuclear weapons.
After iron the binding energy per nucleon begins decreasing, so it is possible for energy to be released if a heavy nucleus breaks apart into two lighter ones. This splitting of atoms is known as nuclear fission. This is the source of energy for nuclear power plants and conventional nuclear bombs like the two that the United States used to destroy the buildings and civilians of Hiroshima and Nagasaki.
Nuclear reactions occur naturally on Earth, and are in fact quite common. These include alpha decay and beta decay, and heavy nuclei such as uranium may also undergo spontanious fission. There is even one known example of a naturally occurring fission reactor, which was active in Oklo, Gabon, Africa over 1.5 billion years ago. 
Production of Heavy Elements
As the Universe cooled after the big bang it eventually became possible for particles as we know them to exist. The most common particles created in the big bang which are still easily observable to us today were protons (hydrogen) and electrons (in equal numbers). Some heavier elements were created as the protons colided with each other, but most of the heavy elements we see today were created inside of stars during a series of fusion stages, such as the proton-proton chain, the CNO cycle and the triple-alpha process. Progressively heavier elements are created during the evolution of a star. Since the binding energy per nucleon peaks around iron, energy is only released in fusion processes occurring below this point. Since the creation of heavier nuclei by fusion costs energy, nature resorts to the process of neutron capture. Neutrons (due to their lack of charge) are readily absorbed by a nucleus. The heavy elements are created by either a slow neutron capture process (the so-called s process) or by the rapid, or r process. The s process occurs in thermally pulsing stars (called AGB, or asymptotic giant branch stars) and takes hundreds to thousands of years to reach the heaviest elements of lead and bismuth. The r process is thought to occur in supernova explosions due to the fact that the conditions of high temperature, high neutron flux and ejected matter are present. These stellar conditions make the sucessive neutron captures very fast, involving very neutron-rich species which then beta-decay to heavier elements, especially at the so-called waiting points that correspond to more stable nuclides with closed neutron shells (magic numbers). The r process duration is typically in the range of a few seconds.
A heavy nucleus can contain hundreds of nucleons (neutrons and protons), which means that to some approximation it can be treated as a classical system, rather than a quantum-mechanical one. In the resulting liquid-drop model, the nucleus has an energy which arises partly from surface tension and partly from electrical repulsion of the protons. The liquid-drop model is able to reproduce many features of nuclei, including the general trend of binding energy with respect to mass number, as well as the phenomenon of nuclear fission.
Superimposed on this classical picture, however, are quantum-mechanical effects, which can be described using the nuclear shell model, developed in large part by Maria Goeppert-Mayer. Nuclei with certain numbers of neutrons and protons (the magic numbers 2, 8, 20, 50, 82, 126, ...) are particularly stable, because their shells are filled.
Much of current research in nuclear physics relates to the study of nuclei under extreme conditions such as high spin and excitation energy. Nuclei may also have extreme shapes (similar to that of american footballs) or extreme neutron-to-proton ratios. Experimenters can create such nuclei using artificially induced fusion or nucleon transfer reactions, employing ion beams from a accelerator. Beams with even higher energies can be used to create nuclei at very high temperatures, and there are signs that these experiments have produced a phase transition from normal nuclear matter to a new state, the quark-gluon plasma, in which the quarks mingle with one another, rather than being segregated in triplets as they are in neutrons and protons.
- List of particles
- nuclear fusion
- nuclear fission
- nuclear medicine
- nuclear physics
- atomic number
- atomic mass
- SCK.CEN Belgian Nuclear Research Centre Mol, Belgium
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