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AtomObscure Definitions
An atom is the smallest irreducible constituent of a chemical system. The word is derived from the Greek atomos, indivisible, from a-, not, and tomos, a cut. It usually means chemical atoms, the basic constituents of molecules and ordinary matter. Atoms are not divisible by chemical reactions but are now known to be composed of even smaller subatomic particles. The diameter of an atom is generally in the range of 10pm to 100pm. The variety of matter that is dealt with in everyday experience consists of discrete atoms. The existence of such particles was first proposed by Greek philosophers such as Democritus, Leucippus, and the Epicureans, but without any real way to be sure, the concept disappeared until it was revived by Rudjer Boscovich in the 18th century, and after that applied to chemistry by John Dalton. Rudjer Boscovich based his theory on Newtonian mechanics and published it in 1758 within his Theoria philosophiae naturalis redacta ad unicam legem virium in natura existentium. According to Boscovich, atoms are stuctureless points, which exhibit repelling and attracting forces on each other, depending on distance. John Dalton used the atomic theory to explain why gases always combine in simple ratios. It was with Amedeo Avogadro's work, in the 19th century, that scientists began to distinguish atoms and molecules. In modern times atoms have been observed experimentally. As it turns out, atoms are themselves made out of smaller particles. In fact, almost all of an atom is empty space. At the center is a tiny positive nucleus composed of nucleons (protons and neutrons), and the rest of the atom contains only the fairly flexible electron shells. Usually atoms are electrically neutral with as many electrons as protons. Atoms are generally classified by the atomic number, which corresponds to the number of protons in the atom. For example, carbon atoms are those atoms containing 6 protons. All atoms with the same atomic number share a wide variety of physical properties and exhibit the same chemical behavior. The various kinds of atoms are listed in the Periodic table. Atoms having the same atomic number, but different atomic masses (due to their different numbers of neutrons), are called isotopes. The simplest atom is the hydrogen atom, having atomic number 1 and consisting of one proton and one electron. It has been the subject of much interest in science, particularly in the early development of quantum theory. The chemical behavior of atoms is largely due to interactions between the electrons. In particular the electrons in the outermost shell, called the valence electrons, have the greatest influence on chemical behavior. Core electrons (those not in the outer shell) play a role, but it is usually in terms of a secondary effect due to screening of the positive charge in the atomic nucleus. There is a strong tendency for atoms to completely fill (or empty) the outer electron shell, which in hydrogen and helium has space for two electrons, and in all other atoms has space for eight. This is achieved either by sharing electrons with neighboring atoms or by completely removing electrons from other atoms. When electrons are shared a covalent bond is formed between the two atoms. Covalent bonds are the strongest type of atomic bond. When one or more electrons are completely removed from one atom by another, ions are formed. Ions are atoms that possess a net charge due to an imbalance in the number of protons and electrons. The ion that stole the electron(s) is called an anion and is negatively charged. The atom that lost the electron(s) is called a cation and is positively charged. Cations and anions are attracted to each other due to coulombic forces between the positive and negative charges. This attraction is called ionic bonding and is weaker than covalent bonding. As mentioned above covalent bonding implies a state in which electrons are shared equally between atoms, while ionic bonding implies that the electrons are completely confined to the anion. Except for a limited number of extreme cases, neither of these pictures is completely accurate. In most cases of covalent bonding, the electron is unequally shared, spending more time around the more electronegative atom, resulting in the covalent bond having some ionic character. Similarly, in ionic bonding the electrons often spend a small fraction of time around the more electropositive atom, resulting in some covalent character for the ionic bond. Models of the atom
This article is adapted from from Wikipedia All Wikipedia article text is available under the terms of the GNU Free Documentation License Mr Tompkins in Paperback : Comprising 'Mr Tompkins in Wonderland' and 'Mr Tompkins Explores the Atom' by George Gamow Eyewitness: Matter by Christopher Cooper The Ghost in the Atom : A Discussion of the Mysteries of Quantum Physics by P. C. W. Davies Atom Archives Vol. 2, The by Gardner Fox Skis Against the Atom: The Exciting, First Hand Account of Heroism and Daring Sabotage During the Nazi Occupation of Norway by Knut Haukelid Stellar Alchemy : The Celestial Origin of Atoms by Michel Cassé The Atom and the Fault: Experts, Earthquakes, and Nuclear Power by Richard Meehan Adventures With Atoms and Molecules: Chemistry Experiments for Young People (Adventures With Science , No 1) by Robert C. Mebane Boltzmanns Atom: The Great Debate That Launched A Revolution In Physics by David Lindley Atoms, Radiation, and Radiation Protection, 2nd Edition by James E. Turner Essential Elements: Atoms, Quarks, and the Periodic Table by Matt Tweed Right Hand, Left Hand : The Origins of Asymmetry in Brains, Bodies, Atoms and Cultures by Chris McManus Photons and Atoms : Introduction to Quantum Electrodynamics by Claude Cohen-Tannoudji What's Smaller Than a Pygmy Shrew? by Robert E. Wells Atom—Photon Interactions : Basic Processes and Applications by Claude Cohen-Tannoudji Recent Atom related patents From USPTO: 6718152: Fully fluorinated polymer coated development electrodes 6718080: Optical programmable delay system 6718002: Method and device for removing radioactive deposits 6717952: Method and apparatus for media data transmission 6717772: Magnetic head slider having protrusions provided on the medium-facing surface and manufacturing method therefor 6717413: Contact potential difference ionization detector 6717358: Cascaded organic electroluminescent devices with improved voltage stability 6717241: Magnetic shielding for integrated circuits 6717221: Method of fabricating MOSFET transistors with multiple threshold voltages by halo compensation and masks 6717199: Tailored insulator properties for devices 6717165: Device and method for detecting the drive state of a turbo pump in a tandetron accelerator of an ion implantation device 6717136: Microfludic system (EDI) 6717134: Calibration method 6717025: Process for removing oxygenates from an olefinic stream 6717020: Method of producing p-hydroxyphenylalkanols 6717018: Production of oxygenated products 6717016: Phosphine compound, transition metal complex containing the same phosphine compound as ligand and asymmetric synthesis catalyst containing the complex 6717013: Perfluoroalkylated aniline compound and process for producing the same 6717003: Silicone compound, a powder surface-treated with this compound, and a makeup containing this powder 6716998: Process for synthesis of 2-yl chroman derivatives 6716995: Design and synthesis of advanced NLO materials for electro-optic applications 6716994: Mobility-Modifying Cyanine Dyes 6716993: Polymethine compound and near-infrared absorbing material comprising same 6716992: Cycloaliphatic epoxy compounds containing styrenic, cinnamyl, or maleimide functionality 6716991: Process for preparing a substituted pyrazolyl benzenesulfonamide for the treatment of inflammation 6716988: Urea substituted imidazopyridines 6716987: Derivatives of benzofuran or benzodioxazole compounds 6716985: Process for making paroxetine 6716983: S type 2-substituted hydroxy-2-indolidinylbutyric ester compounds and process for preparation thereof 6716982: Method for preparing camptothecin derivatives 6716981: Process for the preparation of N-(amino-4, 6-dihalo-pyrimidine) formamides 6716980: Process to produce oxazolidinones 6716979: Derivatives of 1,2-dihydro-7-hydroxyquinolines containing fused rings 6716978: Therapeutically active compounds based on indazole bioisostere replacement of catechol in PDE4 inhibitors 6716976: Process for selective oxidation of cellulose 6716975: Antisense modulation of EDG1 expression 6716969: Reactive dye compounds 6716965: Adipocyte complement related protein zacrp13 6716963: Peptide antiangiogenic drugs 6716961: Chiral peptide nucleic acids with a N-aminoethyl-d-proline backbone 6716958: Waveguide systems or structures or parts thereof, containing polycyanate copolymers prepared from polyfunctional cyanates and fluorinated monocyanates 6716956: Process for preparing polyarylene ethers 6716954: Golf ball formed from a polyisocyanate copolymer and method of making same 6716952: Flame retardant and flame-retardant resin composition containing the same 6716949: Amphipathic polymer particles and methods of manufacturing the same 6716948: Controlled-architecture polymers and use thereof as separation media 6716946: Coating 6716945: Multimodal fluoropolymers and methods of making the same 6716944: Water and oil repellent composition |