Atomic nucleus
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.
Nuclear Makeup
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.
Isotopes
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.
Nuclear Decay
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.
Nucleus Size
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.
History
The discovery of the electron was the first indication that the atom had internal structure. This structure was initially imagined according to the "raisin cookie" or "plum pudding" model, in which the small, negatively charged electrons were embedded in a large sphere containing all the positive charge. Ernest Rutherford and Marsden, however, discovered in their famous 1911 gold foil experiment that alpha particles from a radium source were sometimes scattered backwards from a gold foil, which led to the acceptance of the Bohr model, a planetary model in which the electrons orbited a tiny nucleus in the same way that the planets orbit the sun.
Nuclear Fusion
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.
Nuclear Fission
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. [1]
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.
Nuclear Physics
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.
See also
- List of particles
- radioactivity
- nuclear fusion
- nuclear fission
- nuclear physics
- atomic number
- atomic mass
- isotope
External links
- SCK.CEN Belgian Nuclear Research Centre Mol, Belgium