atmosphere Gaseous envelope that surrounds the Earth. Near the sur¬ face it has a well-defined chemical composition (see air). In addition to gases, the atmosphere contains solid and liquid particles in suspension. Scientists divide the atmosphere into five main layers: in ascending order, the troposphere (surface to 6-8 mi, or 10-13 km); the stratosphere (4-11 mi, or 6-17 km, to about 30 mi, or 50 km); the mesosphere (31-50 mi, or 50-80 km); the thermosphere (50-300 mi, or 80-480 km); and the exosphere (from 300 mi and gradually dissipating). Most of the atmo¬ sphere consists of neutral atoms and molecules, but in the ionosphere a significant fraction is electrically charged. The ionosphere begins near the top of the stratosphere but is most distinct in the thermosphere. See also ozone layer. See illustration opposite.
atmospheric pressure or barometric pressure Force per unit area exerted by the air above the surface of the Earth. Standard sea-level pressure, by definition, equals 1 atmosphere (atm), or 29.92 in. (760 mm) of mercury, 14.70 lbs per square in., or 101.35 kilopascals, but pressure varies with elevation and temperature. It is usually measured with a mer¬ cury barometer (hence the term barometric pressure), which indicates the height of a column of mercury that exactly balances the weight of the
column of atmosphere above it. It may also be measured using an aner¬ oid barometer, in which the action of atmospheric pressure in bending a metallic surface is made to move a pointer.
atoll \'a- ,tol\ Coral reef enclosing a lagoon. Atolls consist of ribbons of reef that may not be circular but that are closed shapes, sometimes miles across, around a lagoon that may be 160 ft (50 m) deep or more. Most of the reef itself is usually below the water surface; around the rim along the top are usually low, flat islands or more continuous strips of low, flat land.
atom Smallest unit into which matter can be divided and still retain the characteristic properties of an element. The word derives from the Greek atomos (“indivisible”), and the atom was believed to be indivisible until the early 20th century, when electrons and the nucleus were discovered. It is now known that an atom has a positively charged nucleus that makes up more than 99.9% of the atom’s mass but only about 1/100,000 of its volume. The nucleus is composed of positively charged protons and elec¬ trically neutral neutrons, each about 2,000 times as massive as an elec¬ tron. Most of the atom’s volume consists of a cloud of electrons that have very small mass and negative charge. The electron cloud is bound to the nucleus by the attraction of opposite charges. In a neutral atom, the pro¬ tons in the nucleus are balanced by the electrons. An atom that has gained or lost electrons becomes negatively or positively charged and is called an ion. See illustration on opposite page.
atomic bomb Weapon whose great explosive power results from the sudden release of energy upon the splitting, or fission, of the nuclei of heavy elements such as plutonium or uranium (see nuclear fission). With
(°C)
-100 -80 -60 -40 -20 0 20
pressure (atm)
In Earth's atmosphere, the limits of the atmospheric layers are approximate and variable, especially with latitude. Most weather occurs within the troposphere. The ozone layer, which absorbs most incoming ultraviolet radiation, forms part of the stratosphere. The thermosphere extends hundreds of miles above Earth's surface and is bounded by outer space. Atmospheric pressure drops off steadily with alti¬ tude, but temperature rises and falls through successive layers in a more complex manner.
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© 2006 Encyclopaedia Britannica, Inc.
atomic number ► atonement I 127
nucleus —
neutron
-S
J proton
e electron
The classical "planetary" model of an atom. The protons and neutrons in the nucleus are circled by electrons in "orbit" around the nucleus. The number of pro¬ tons determines which element is represented, the number of electrons determines its charge, and the number of neutrons determines which isotope of the element is represented.
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only 11-33 lb (5-15 kg) of highly enriched uranium, a modern atomic bomb could generate a 15-kiloton explosion, creating a huge fireball, a large shock wave, and lethal radioac¬ tive fallout. The first atomic bomb, developed by the Manhattan Project during World War II, was set off on July 16, 1945, in the New Mexico desert. The only atomic bombs used in war were dropped by the U.S. on Hiroshima on Aug. 6, 1945, and on Nagasaki three days later. In 1949 the Soviet Union tested its first atomic bomb, followed by Britain (1952),
France (1960), China (1964), India (1974), and Pakistan (1998). Israel and South Africa were suspected of testing atomic weapons in 1979. See also HYDROGEN BOMB; NUCLEAR NON¬ PROLIFERATION Treaty; nuclear weapon.
atomic number Number of a chemical element in the systematic, ordered sequence shown in the periodic table. The elements are arranged in order of increasing number of protons in the nucleus of the atom (the same as the number of electrons in the neutral atom), and that number for each element is its atomic number.
atomic physics Scientific study of the structure of the atom, its energy states, and its interaction with other particles and fields. The modem understanding of the atom is that it consists of a heavy nucleus of posi¬ tive charge surrounded by a cloud of light, negatively charged electrons. The physical properties of atoms are largely determined by the laws of quantum mechanics and quantum electrodynamics. The primary tools for the study of these properties are spectroscopy, particle collisions (see particle accelerator), and statistical models that simulate complex, many-body interactions (such as gas dynamics). A broad field, atomic physics has applications in the study of condensed matter, gases, chemical-reaction
First atomic bomb test, near Alamo¬ gordo, New Mexico, July 16, 1945.
COURTESY OF LOS ALAMOS NATIONAL LABORATORY, NEW MEXICO
mechanisms, atmospheric science, lasers, nuclear physics, and the arrange¬ ment of elements in the periodic table.
atomic weapon See nuclear weapon
atomic weight Ratio of the average mass of a chemical element’s atoms to Vn the mass of an atom of the carbon-12 isotope. The original standard of atomic weight, established in the 19th century, was hydrogen, with a value of 1. From c. 1900 until 1961, the reference standard was oxygen, with a value of 16, and the unit of atomic mass was defined as Vie the mass of an oxygen atom. Oxygen, however, contains small amounts of two isotopes that are heavier than the most abundant one, and 16 is actu¬ ally a weighted average of the masses of the three isotopes of oxygen. Therefore, the standard was changed to one based on carbon-12. The new scale required only minimal changes to the values that had been used for chemical atomic weights.
atomism Philosophical doctrine that material objects are aggregates of simpler parts known as atoms. Atomism in the strict sense is character¬ ized by three points: the atoms are absolutely indivisible, qualitatively identical apart from shape, size, and motion, and combinable with each other only by juxtaposition. Atomism is usually associated with realism and mechanism; it is mechanistic because it maintains that all observable changes can be reduced to changes in the configuration of the atoms that constitute matter. It is opposed to holism because it holds that the prop¬ erties of any whole can be explained in terms of those of its parts.
Aton or Aten Va-ton, 'a-ton\ In ancient Egyptian religion, a sun god, depicted as the solar disk emitting rays terminating in human hands. The pharaoh Akhenaton (r. 1353-36 bc) declared Aton to be the only god, and in opposition to the Amon-Re priesthood of Thebes, built the city of Akhetaton as the center for Aton’s worship, but Aton’s religion is poorly understood. After Akhenaton’s death, the old religion was restored.