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The observation that many atomic weights are close to whole numbers led the British chemist William Prout to suggest in 1816 that all elements might be composed of hydrogen atoms. Subsequent measurements of atomic weights revealed that chlorine, for example, has an atomic weight of 35.455. The discovery of such fractional atomic weights appeared to invalidate Prout's hypothesis until a century later, when it was discovered that the atoms of most elements do not all have the same weight. Atoms of the same element that differ in weight are known as isotopes. In the case of chlorine two isotopes occur in nature. Experiments show that chlorine is a mixture of three parts of chlorine-35 for every one part of the heavier chlorine-37 isotope. This proportion accounts for the observed atomic weight of chlorine. Atomic scientists can measure isotopes with great precision. For example, the light isotope of chlorine is measured at 34.97867 amu.
The standard used for the calculation of atomic weights has recently been changed. During the first part of the 20th century it was customary to use natural oxygen as the standard against which atomic weights or masses were computed; oxygen was assigned an integral atomic weight of 16. This standard was used by chemists even after the rare isotopes of oxygen (oxygen-17 and oxygen-18) were discovered in 1929, because the small amounts of these isotopes in natural oxygen are relatively, although not absolutely, in constant proportion to the abundant isotope, oxygen-16. Physicists found it easier, however, to compute atomic masses against only the oxygen-16 isotope. This method resulted in two slightly different tables of atomic weights or masses. The situation was resolved in the early 1960s, when the international unions of chemistry and physics agreed on a single new standard, the abundant isotope of carbon, carbon-12. The new standard completely replaced the two earlier standards for all scientists. The new standard is particularly appropriate because carbon-12 is often used as a reference standard in computations of atomic masses using the mass spectrometer. Moreover, the table of atomic weights based on carbon-12 is in close agreement with the old table based on natural oxygen.
It is convenient to visualize the electrons moving about the nucleus of an atom much as if they were planets moving about the sun. This view is much more precise than that held by contemporary physicists, however. It is now known that it is impossible to pinpoint the precise position of an electron in the atom's space without disturbing its predicted location at some future time. This uncertainty is resolved by attributing to the atom a cloudlike form, in which the electron's position is defined in terms of the probability of finding it at some distance from the nucleus. This rather fuzzy schematic conception of the atom may be reconciled with the solar-system model by noting that in the tiny space of the atom the electron, which makes many billions of orbits around the nucleus in a single second, is everywhere at once. The cloud view thus gives a form to the atom that is not supplied by a solar-system model.
An electrostatic force keeps the electrons from flying off. The positive charges from the nucleus hold the electrons at very precise energy levels (orbitals). A certain energy level can hold a maximum of electrons. Bohr proposed that this maximum number of electrons per energy level can be calculated mathematically by 2n2, where n is the number of the level. For example, the 3e energy level can contain just 18 electrons.
It is important to understand that electrons from lower energy levels are held in more firmly than electrons from the higher levels. However, if the atom is exposed to an exterior energy source, the electrons from inferior energy levels can jump up to a higher level. The electrons jump from one level to another when they absorb energy.
1: Electron at the fundamental level
2: A form of energy is absorbed by the electron
3: The electron jumps to a higher energy level
4: The electron returns to its fundamental state
5: The emission of electromagnetic radiation
Because this increase in energy in momentary, the electron cannot stay at a level that is too high. It will return to its fundamental level. During this return of the electron to its fundamental level, there will be an emission of radiation.
In 1919 Rutherford exposed nitrogen gas to a radioactive source that emitted alpha particles. Some of the alpha particles collided with the nuclei of the nitrogen atoms. As a result of these collisions, the nitrogen atoms were transmuted into oxygen atoms. A positively charged particle was emitted from the nucleus of each of the atoms undergoing transmutation. These particles were recognized as being identical to the nuclei of hydrogen atoms. They are called protons (see Proton). Although further research proved that protons are constituents of the nuclei of all elements, no more clues to the structure of the nucleus were found until 1932, when the British physicist Sir James Chadwick discovered in the nucleus another particle, known as the neutron, having the same weight as the proton but without an electrical charge. It was then realized that the nucleus is made up of protons and neutrons. In any given atom, the number of protons is equal to the number of electrons and hence to the atomic number of the atom. Isotopes are then explained as atoms of the same element (that is, containing the same number of protons) that have different numbers of neutrons. In the case of chlorine, one isotope is identified by the symbol of 35Cl and its heavy relative by 37Cl. The superscripts identify the mass number of the isotope and are numerically equal to the total number of neutrons and protons in the nucleus of the atom. Sometimes the atomic number is given as a subscript.
The least stable arrangement of nuclei is one in which an odd number of neutrons and an odd number of protons are present; all but four isotopes containing nuclei of this kind are radioactive. The presence of a large excess of neutrons over protons detracts from the stability of a nucleus; nuclei in all isotopes of elements above bismuth in the periodic table contain this type of arrangement, and they are all radioactive. Most known stable nuclei contain an even number of protons and an even number of neutrons.
This subnuclear world was first revealed in cosmic rays (see Cosmic Rays). These rays consist of highly energetic particles that constantly bombard the earth from outer space, penetrating down through the atmosphere and even into the earth's crust. Cosmic radiation includes many types of particles, some having energies far exceeding anything achieved in particle accelerators. When these energetic particles strike nuclei, new particles are created. Among the first such particles to be observed were the muons (detected in 1937) and pions (1947). The existence of the pion had been predicted in 1935 by the Japanese physicist Yukawa Hideki.
According to the most widely accepted theory, nuclear particles are held together by "exchange forces," in which pions common to both neutrons and protons are continuously exchanged between them. The binding of protons and neutrons by pions is similar to the binding of two atoms in a molecule through sharing or exchanging a common pair of electrons. These particles are about 200 times as heavy as electrons. The muon is essentially a heavy electron and can be either positively or negatively charged. The pion, slightly heavier than the muon, can carry a positive or negative charge, or no charge.
Accelerator studies eventually established that each kind of particle also has an antiparticle of the same mass but opposite in charge or other electromagnetic property. Physicists have long sought a theory that would put this bewildering array of particles in order. Particles are now grouped according to the force that usually controls their interactions. Hadrons (strong nuclear force) include hyperons, mesons, and the neutron and proton. Leptons (electromagnetic and weak forces) include the tau, muon, electron, and neutrinos. Bosons (particlelike objects associated with interactions) include the photon and the hypothetical carriers of the weak force and of gravitation. The weak nuclear force is evident in such radioactive or particle-decay reactions as alpha decay (the release of a helium nucleus from an unstable atomic nucleus). (see Antimatter).
In 1963 the U.S. physicists Murray Gell-Mann and George Zweig proposed that hadrons are actually combinations of more fundamental particles called quarks, the interactions of which are carried by particlelike gluons. This theory underlies current investigations and has served to predict the existence of further particles.
Thermonuclear fusion occurs in stars, including the sun, and is the source of their heat and light. Uncontrolled fusion is seen in the explosion of a hydrogen bomb, but physicists are currently trying to develop a practical controlled-fusion device.
The antiproton, the antiparticle of the proton, is also called a negative proton. It differs from the proton in having a negative charge and not being a constituent of atomic nuclei. The antiproton is stable in a vacuum and does not decay spontaneously. When an antiproton collides with a proton or a neutron, however, the two particles are transformed into mesons, which have an extremely short half-life. Although physicists had postulated the existence of this elementary particle since the 1930s, the antiproton was positively identified for the first time in 1955 at the University of California Lawrence Berkeley National Laboratory.
Because protons are essential parts of ordinary matter, they are obviously stable. Particle physicists are nevertheless interested in learning whether protons eventually decay after all, on a time scale of many billions of billions of years. This interest derives from current attempts at grand unification theories that would combine all four fundamental interactions of matter in a single scheme. Many of these attempts call for the ultimate instability of the proton, so research groups at a number of accelerator facilities are conducting tests to detect such decays. By the end of the 1980s no clear evidence had yet been found; possible results thus far can also be interpreted in other ways.
About 87 percent of cosmic rays are protons (hydrogen nuclei), and about 12 percent are alpha particles (helium nuclei). Heavier elements are also present, but in greatly reduced numbers. For convenience, scientists divide the elements into light (lithium, beryllium, and boron), medium (carbon, nitrogen, oxygen, and fluorine), and heavy (the remainder of the elements). The light elements compose 0.25 percent of the cosmic rays. Because the light elements constitute only about 1 billionth of all matter in the universe, it is believed that light-element cosmic rays are formed by the fragmentation of heavier cosmic rays that collide with protons, as they must do in traversing interstellar space. From the abundance of light elements in cosmic rays, it is inferred that cosmic rays have passed through material equivalent to a layer of water 4 cm (about 1.5 in) thick. The medium elements are increased by a factor of about 10 and the heavy elements by a factor of about 100 over normal matter, suggesting that at least the initial stages of acceleration to the observed energies occur in regions enriched in heavy elements.
Energies of cosmic-ray particles are measured in units of giga (billion) electron volts (GeV) per proton or neutron in the nucleus. The distribution of proton energies of cosmic rays peaks at 0.3 GeV, corresponding to a velocity two-thirds that of light, and falls toward higher energies, although particles up to 1011 GeV have been detected through showers of secondary particles created when they collide with atmospheric nuclei. About 1 electron volt of energy per cubic centimeter of space is invested in cosmic rays in our galaxy, on the average.
Even an extremely weak magnetic field deflects cosmic rays from straight-line paths; a field of 3 × 10-6 gauss, such as is believed to be present throughout interstellar space, is sufficient to force a 1-GeV proton to gyrate with a radius of 10-6 light-year. A 1011-GeV particle gyrates with a radius of 105 light-years, about the size of the Galaxy. Thus, the interstellar magnetic field prevents cosmic rays from reaching the earth directly from their points of origin, accounting for the directions of arrival being isotropically distributed at even the highest energies.
In the 1950s, radio emission from the Milky Way galaxy was discovered and interpreted as radiation from energetic electrons gyrating in interstellar magnetic fields. The intensity of the electron component of cosmic rays, about 1 percent of the intensity of the protons at the same energy, agrees with the value inferred for interstellar space in general from the radio emission.
Radio astronomical studies of other galaxies show that they also contain energetic electrons. The nuclei of some galaxies are far more luminous than the Milky Way in radio waves, indicating that sources of energetic particles are located there. The physical mechanism producing these particles is not known.
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