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In chemistry and physics, an atom (Greek άτομον meaning "indivisible") is the smallest possible particle of a chemical element that retains its chemical properties. The word atom may also refer to the smallest possible indivisible fundamental particle. This definition must not be confused with that of chemical atoms, since chemical atoms (hereafter "atoms") are composed of smaller subatomic particles.
Most atoms are composed of three types of subatomic particles which govern their external properties:
- electrons, which have a negative charge and are the least massive of the three;
- protons, which have a positive charge and are about 1836 times more massive than electrons; and
- neutrons, which have no charge and are about 1838 times more massive than electrons.
Protons and neutrons are both nucleons and make up the dense, massive atomic nucleus. The electrons form the much larger electron cloud surrounding the nucleus.
Atoms differ in the number of each of the subatomic particles they contain. The number of protons in an atom (called the atomic number) determines the element of the atom. Within a single element, the number of neutrons may also vary, determining the isotope of that element. Atoms are electrically neutral if they have an equal number of protons and electrons. Electrons that are furthest from the nucleus may be transferred to other nearby atoms or even shared between atoms. Atoms which have either a deficit or a surplus of electrons are called ions. The number of protons and neutrons in the atomic nucleus may also change, via nuclear fusion, nuclear fission or radioactive decay.
Atoms are the fundamental building blocks of chemistry, and are conserved in chemical reactions. Atoms are able to bond into molecules and other types of chemical compounds. Molecules are made up of multiple atoms; for example, a molecule of water is a combination of two hydrogen atoms and one oxygen atom.
Bold text POOPY
Atoms and antimatter
- see main article antimatter
Antimatter can also form atoms, composed of positrons, antiprotons, and antineutrons. Since antimatter is very difficult to produce and store, only a small amount antihydrogen has ever existed on Earth. This was produced at CERN in the ATHENA and ATRAP experiments using the Antiproton Decelerator.
Atoms and the Big Bang
In models of the Big Bang, Big Bang nucleosynthesis predicts that within one to three minutes of the Big Bang almost all atomic material in the universe was created. During this process, nuclei of hydrogen and helium formed abundantly, but almost no elements heavier than lithium. Hydrogen makes up approximately 75% of the atoms in the universe; helium makes up 24%; and all other elements make up just 1%. However, although nuclei (fully-ionized atoms) were created, neutral atoms themselves could not form in the intense heat.
Big Bang chronology of the atom continues to approximately 379,000 years after the Big Bang when the cosmic temperature had dropped to just 3,000 K. It was then cool enough to allow the nuclei to capture electrons. This process is called recombination, during which the first neutral atoms took form. Once atoms become neutral, they only absorb photons of a discrete absorption spectrum. This allows most of the photons in the universe to travel unimpeded for billions of years. These photons are still detectable today in the cosmic microwave background.
After Big Bang nucleosynthesis, no heavier elements could be created until the formation of the first stars. These stars fused heavier elements through stellar nucleosynthesis during their lives and through supernova nucleosynthesis as they died. The seeding of the interstellar medium by heavy elements eventually allowed the formation of terrestrial planets like the Earth.
History of atomic theory
Early atomism
From the 6th century BC, Hindu, Buddhist and Jaina philosophers in ancient India developed the earliest atomic theories. The first philosopher who formulated ideas about the atom in a systematic manner was Kanada who lived in the 6th century BC. Another Indian philosopher, Pakudha Katyayana who also lived in the 6th century BC and was a contemporary of Gautama Buddha, had also propounded ideas about the atomic constitution of the material world. Indian atomists believed that an atom could be one of upto six elements, with each element having upto 24 properties. They developed detailed theories of how atoms could combine, react, vibrate, move, and perform other actions, and had particularly elaborate theories of how atoms combine, which explains how atoms first combine in pairs, and then group into trios of pairs, which are the smallest visible units of matter. This parallels with the structure of modern atomic theory, in which pairs or triplets of supposedly fundamental quarks combine to create most typical forms of matter. They had also suggested the possibility of splitting an atom which, as we know today, is the source of atomic energy. (See Indian atomism for more details.)
Democritus and Leucippus, Greek philosophers in the 5th century BC, presented a theory of atoms. (See Atomism for more details.) The Greeks believed that atoms were all made of the same material but had different shapes and sizes, which determined the physical properties of the material. For instance, the atoms of a liquid were thought to be smooth, allowing them to slide over each other. None of these ideas, however, were founded in scientific experimentation.
During the Middle Ages (the Islamic Golden Age), Islamic atomists develop atomic theories that represent a synthesis of both Greek and Indian atomism. (See Islamic atomism for more details.) Older Greek and Indian ideas were further developed by Islamic atomists, along with new Islamic ideas, such as the possibility of there being particles smaller than an atom. As Islamic influence began spreading through Europe, the ideas of Islamic atomism, along with the older ideas of Greek and Indian atomism, spread throughout Europe by the end of the Middle Ages, where modern atomic theories began taking shape.
Birth of modern atomic theory
In 1808, John Dalton proposed that an element is composed of atoms of a single, unique type, and that although their shape and structure was immutable, atoms of different elements could combine to form more complex structures (chemical compounds). He deduced this after the experimental discovery of the law of multiple proportions — that is, if two elements form more than one compound between them, then the ratios of the masses of the second element which combine with a fixed mass of the first element will be ratios of small whole numbers.
The experiment in question involved combining nitrous oxide (NO) with oxygen (O2). In one combination, these gases formed dinitrogen trioxide (N2O3), but when he repeated the combination with double the amount of oxygen (a ratio of 1:2), they instead formed nitrogen dioxide (NO2).
4NO + O2 → 2N2O3
4NO + 2O2 → 4NO2
Atomic theory conflicted with the theory of infinite divisibility, which states that matter can always be divided into smaller parts. In 1827, biologist Robert Brown observed that pollen grains floating in water constantly jiggled about for no apparent reason. In 1905, Albert Einstein theorised that this Brownian motion was caused by the water molecules continuously knocking the grains about, and developed a mathematical theory around it. This theory was validated experimentally in 1911 by French physicist Jean Perrin.
Discovery of subatomic particles
For much of this time, atoms were thought to be the smallest possible division of matter. However, in 1897, J.J. Thomson published his work proving that cathode rays are made of negatively charged particles (electrons). Since cathode rays are emitted from matter, this proved that atoms are made up of subatomic particles and are therefore divisible, and not the indivisible atomos postulated by Democritus. Physicists later invented a new term for such indivisible units, "elementary particles", since the word atom had come into its common modern use.
Study of atomic structure
At first, it was believed that the electrons were distributed more or less uniformly in a sea of positive charge (the plum pudding model). However, an experiment conducted in 1909 by colleagues of Ernest Rutherford demonstrated that atoms have a most of their mass and positive charge concentrated in a nucleus. In the gold foil experiment, alpha particles (emitted by polonium) were shot through a sheet of gold. Rutherford observed that most of the particles passed straight through the sheet with little deflection (striking a fluorescent screen on the other side). About 1 in 8000 of the alpha particles, however, were heavily deflected (by more than 90 degrees). This led to the planetary model of the atom in which pointlike electrons orbited in the space around a massive compact nucleus like planets orbiting the Sun.
The nucleus was later discovered to contain protons, and further experimentation by Rutherford found that the nuclear mass of most atoms surpassed that of the protons it possessed; this led him to postulate the existence of neutrons, whose existence would be proven in 1932 by James Chadwick.
The planetary model of the atom still had shortcomings. Firstly, a moving electric charge emits electromagnetic waves; according to classical electromagnetism, an orbiting charge would steadily lose energy and spiral towards the nucleus, colliding with it in a tiny fraction of a second. Secondly, the model did not explain why excited atoms emit light only in certain discrete spectra.
Quantum theory revolutionized physics at the beginning of the 20th century when Max Planck and Albert Einstein postulated that light energy is emitted or absorbed in fixed amounts known as quanta. In 1913, Niels Bohr used this idea in his Bohr model of the atom, in which the electrons could only orbit the nucleus in particular circular orbits with fixed angular momentum and energy. They were not allowed to spiral into the nucleus, because they could not lose energy in a continuous manner; they could only make quantum leaps between fixed energy levels. Bohr's model was extended by Arnold Sommerfeld in 1916 to include elliptical orbits, using a quantization of generalized momentum.
The ad hoc Bohr-Sommerfeld model was extremely difficult to use, but it made impressive predictions in agreement with certain spectral properties. However, the model was unable to explain multielectron atoms, predict transition rates or describe fine and hyperfine structure. In 1925, Erwin Schrödinger developed a full theory of quantum mechanics, described by the Schrödinger equation. Together with Wolfgang Pauli's exclusion principle, this allowed study of atoms with great precision when digital computers became available. Even today, these theories are used in the Hartree-Fock quantum chemical method to determine the energy levels of atoms. Further refinements of quantum theory such as the Dirac equation and quantum field theory made smaller impacts on the theory of atoms.
Another model of historical interest, proposed by Gilbert N. Lewis in 1916, had cubical atoms with electrons statically held at the corners. The cubes could share edges or faces to form chemical bonds. This model was created to account for chemical phenomena such as bonding, rather than physical phenomena such as atomic spectra.
See also
References
- Kenneth S. Krane, Introductory Nuclear Physics (1987)