The smallest part of any material that cannot be broken up by chemical means. Each atom has a center (the nucleus) which contains protons and neutrons. Electrons orbit around the nucleus. The atom is primarily empty space. If the nucleus of an atom were the size of the button on a baseball pitcher's cap, the electrons would be like dust particles revolving around the outside of the baseball stadium.
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Once considered the ultimate particle of an element, believed to be as indivisible as its name indicates. Discovery of radioactivity demonstrated the existence of subatomic particles, notably protons, neutrons, and electrons, the first two comprising most of the mass of the atomic nucleus. We now know that subatomic particles are further classified into hadrons, leptons, and quarks. [G. atomos, indivisible, uncut]
- activated a. an a. possessing more than normal energy as a result of input of energy. SEE ALSO: excited state. SYN: excited a..
- Bohr a. a concept or model of the a. in which the negatively charged electrons move in circular or elliptical orbits around the positively charged nucleus, energy being emitted or absorbed when electrons change from one orbit to another.
- excited a. SYN: activated a..
- ionized a. an a. that possesses an electrostatic charge as a result of loss or gain of electrons; e.g., H+, Ca2+, Cl, O2−.
- labeled a. a radioactive a., or a stable but rare one, which by its presence in a molecule helps localization or measurement of that molecule. SYN: tagged a..
- nuclear a. a concept or model of the a. characterized by the presence of a small, massive nucleus at its center.
- quaternary carbon a. an a. of carbon to which four other carbon atoms are attached.
- radioactive a. an a. with an unstable nucleus, which emits particulate or electromagnetic radiation (radioactive emission) to achieve greater stability. See radionuclide, half-life, Becquerel.
- recoil a. the remainder of an a. from which a nuclear particle has been emitted or ejected at high velocity; the remainder recoils with a velocity inversely proportional to its mass.
- stripped a. an a. minus all its electrons; a nucleus.
- tagged a. SYN: labeled a..

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at·om 'at-əm n the smallest particle of an element that can exist either alone or in combination
atom·ic ə-'täm-ik adj
atom·i·cal·ly -i-k(ə-)lē adv

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at·om (atґəm) [Gr. atomos indivisible] any one of the ultimate particles of a molecule or of any matter. An atom is the smallest particle of an element that is capable of entering into a chemical reaction. The atom consists of a minute central nucleus, in which practically all of the mass of the atom is concentrated, and of surrounding electrons. The nucleus is positively charged; the amount of the charge corresponds to the atomic number of the atom. See Appendix 5. In a neutral atom the surrounding negative electrons are equal in number to the positive charges on the nucleus. The number and arrangement of these electrons determine all the properties of the atom except its atomic weight and its radioactivity. atomic adj

Medical dictionary. 2011.

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  • Atom — См. Атом Термины атомной энергетики. Концерн Росэнергоатом, 2010 …   Термины атомной энергетики

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  • Atom — Atom …   Deutsch Wörterbuch

  • Atom — • Primarily, the smallest particle of matter which can exist Catholic Encyclopedia. Kevin Knight. 2006. Atom     Atom     † …   Catholic encyclopedia

  • ATOM — Pour les articles homonymes, voir Atom (homonymie). Atom Extension de fichier …   Wikipédia en Français

  • Atom — [a to:m], das; s, e: kleinstes, mit chemischen Mitteln nicht weiter zerlegbares Teilchen eines chemischen Grundstoffes: Atome sind elektrisch neutral. Zus.: Wasserstoffatom. * * * Atom 〈n. 11; Chem.〉 1. kleinstes, mit chem. Mitteln nicht mehr… …   Universal-Lexikon

  • atom — ATÓM, atomi, s.m. 1. Cea mai mică parte dintr un element chimic care mai păstrează însuşirile chimice ale elementului respectiv. ♢ (fiz.; în compusul) Atom gram = greutatea exprimată în grame a masei unui atom. 2. Corpuscul infinit de mic,… …   Dicționar Român

  • Atom — Sn kleinstes Teilchen std. (15. Jh., Form 19. Jh.) Entlehnung. Entlehnt aus l. atomus f., zunächst mit lateinischer Flexion und maskulinem Genus. Das lateinische Wort wiederum ist entlehnt gr. átomos, einer Substantivierung von gr. átomos… …   Etymologisches Wörterbuch der deutschen sprache

  • atom — [at′əm] n. [ME attome < OFr atome < L atomus < Gr atomos, uncut, indivisible, atom < a , not + tomos < temnein, to cut: see TOMY] 1. Obs. any of the indivisible particles postulated by philosophers as the basic component of all… …   English World dictionary

  • Atom — At om, n. [L. atomus, Gr. ?, uncut, indivisible; a priv. + ?, verbal adj. of ? to cut: cf. F. atome. See {Tome}.] 1. (Physics) (a) An ultimate indivisible particle of matter. (b) An ultimate particle of matter not necessarily indivisible; a… …   The Collaborative International Dictionary of English

  • Atom — Atom, von ἄτoμoν, unteilbar, bezeichnete im Sinne der altgriechischen Philosophen die kleinsten Teilchen der Materie. In den Händen der Chemiker hat sich der Begriff des Atoms dahin umgestaltet, daß es, für jedes… …   Lexikon der gesamten Technik

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