atomic weight

Ratio of the average mass of a chemical element's atoms to 112 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 circa 1900 until 1961, the reference standard was oxygen, with a value of 16, and the unit of atomic mass was defined as 116 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 actually 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.

Learn more about atomic weight with a free trial on Britannica.com.

or atomic weapon or thermonuclear weapon

Bomb or other warhead that derives its force from nuclear fission, nuclear fusion, or both and is delivered by an aircraft, missile, or other system. Fission weapons, commonly known as atomic bombs, release energy by splitting the nuclei of uranium or plutonium atoms; fusion weapons, known as hydrogen bombs or thermonuclear bombs, fuse nuclei of the hydrogen isotopes tritium or deuterium. Most nuclear weapons actually combine both processes. Nuclear weapons are the most potent explosive devices ever invented. Their destructive effects include not only a blast equivalent to thousands of tons of TNT but also blinding light, searing heat, and lethal radioactive fallout. The number of nuclear weapons reached a peak of some 32,000 for the United States in 1966 and some 33,000 for the Soviet Union in 1988. Since the end of the Cold War, both countries have decommissioned or dismantled thousands of warheads. Other declared nuclear powers are the United Kingdom, France, China, India, Pakistan, and North Korea. Israel is widely assumed to possess nuclear weapons. Some countries, such as South Africa, Brazil, Argentina, and Iraq, have acknowledged pursuing nuclear weapons in the past but have abandoned their programs. Seealso Nuclear Non-proliferation Treaty; Nuclear Test-Ban Treaty.

Learn more about nuclear weapon with a free trial on Britannica.com.

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 99.9percnt of the atom's mass but only about 1/100,000 of its volume. The nucleus is composed of positively charged protons and electrically neutral neutrons, each about 2,000 times as massive as an electron. 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 protons 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.

Learn more about atom with a free trial on Britannica.com.

or atomic energy

Energy released from atomic nuclei in significant amounts. In 1919 Ernest Rutherford discovered that alpha rays could split the nucleus of an atom. This led ultimately to the discovery of the neutron and the release of huge amounts of energy by the process of nuclear fission. Nuclear energy is also released as a result of nuclear fusion. The release of nuclear energy can be controlled or uncontrolled. Nuclear reactors carefully control the release of energy, whereas the energy release of a nuclear weapon or resulting from a core meltdown in a nuclear reactor is uncontrolled. Seealso chain reaction, nuclear power, radioactivity.

Learn more about nuclear energy with a free trial on Britannica.com.

Scientific study of the structure of the atom, its energy states, and its interaction with other particles and fields. The modern understanding of the atom is that it consists of a heavy nucleus of positive 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 mechanisms, atmospheric science, lasers, nuclear physics, and the arrangement of elements in the periodic table.

Learn more about atomic physics with a free trial on Britannica.com.

Mathematical expression, called a wave function, that describes properties characteristic of no more than two electrons near an atomic nucleus or molecule. An orbital can be considered a three-dimensional region in which there is a 95percnt probability of finding an electron. Atomic orbitals are designated by a combination of numerals and letters (e.g., 1math.s, 2math.p, 3math.d, 4math.f). The numerals are the principal quantum number and are related to the atomic energy level and distance from the nucleus; the letters indicate the orbital's angular momentum and hence its shape. An math.s orbital with zero net orbital angular momentum is spherical. A math.p orbital with one fundamental unit of angular momentum, planck, is shaped somewhat like a dumbbell (planck is Planck's constant, h, divided by 2π). The shapes of the other orbitals are more complicated. Molecular orbitals have geometries determined by the overlap of two or more atomic orbitals and are designated by Greek symbols, e.g., σ and π.

Learn more about orbital with a free trial on Britannica.com.

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.

Learn more about atomic number with a free trial on Britannica.com.

Process by which nuclear reactions between light elements form heavier ones, releasing huge amounts of energy. In 1939 Hans Bethe suggested that the energy output of the sun and other stars is a result of fusion reactions among hydrogen nuclei. In the early 1950s American scientists produced the hydrogen bomb by inducing fusion reactions in a mixture of the hydrogen isotopes deuterium and tritium, forming a heavier helium nucleus. Though fusion is common in the sun and other stars, it is difficult to produce artificially and is very difficult to control. If controlled nuclear fusion is achieved, it might provide an inexpensive energy source because the primary fuel, deuterium, can be extracted from ordinary water, and eight gallons of water could provide the energy equivalent to 2,500 gallons of gasoline.

Learn more about nuclear fusion with a free trial on Britannica.com.

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 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 radioactive 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. Seealso hydrogen bomb; Nuclear Non-proliferation Treaty; nuclear weapon.

Learn more about atomic bomb with a free trial on Britannica.com.

International organization officially founded in 1957 to promote the peaceful use of nuclear energy. Based in Vienna, its activities include research on the applicability of nuclear energy to medicine, agriculture, water resources, and industry; provision of technical assistance; development of radiation safeguards; and public relations programs. Following the Persian Gulf War, IAEA inspectors were called on to certify that Iraq was not manufacturing nuclear weapons. The IAEA and its director general, Mohamed ElBaradei, were awarded the Nobel Prize for Peace in 2005.

Learn more about International Atomic Energy Agency (IAEA) with a free trial on Britannica.com.

International organization established in 1958 to form a common market for developing peaceful uses of atomic energy. It originally had six members; it now includes all members of the European Union. Among its aims were to facilitate the establishment of a nuclear energy industry on a European rather than a national scale, coordinate research, encourage construction of power plants, establish safety regulations, and establish a common market for trade in nuclear equipment and materials. In 1967 its governing bodies were merged into the European Community.

Learn more about European Atomic Energy Community (Euratom) with a free trial on Britannica.com.

An atom is the smallest particle of a chemical element that retains its chemical properties.

In music:

• Atomic (band), a Norwegian jazz quintet
• Atomic (album), an album by Lit
• "Atomic" (song), a song by Blondie
• "Atomic", a song by Tiger Army from Tiger Army III: Ghost Tigers Rise

In other uses:

• Atomic (magazine), an Australian computing and technology magazine
• Atomic Skis, an Austrian ski producer

See also

• Atom (disambiguation)
• Atomicity
• Nuclear
• Atomism, philosophy about the basic building blocks of reality
• Atomic number, the number of protons found in the nucleus of an atom
• Atomic formula, a formula without subformulas
• Atomic Kitten, an English all-female pop group
• Atomic Rooster, a British progressive rock band
• Atomic chess, a chess variant
• Atomic operation, in computer science
• Nuclear weapon
• Nuclear power