atom

[at-uhm]
noun
1.
Physics.
a.
the smallest component of an element having the chemical properties of the element, consisting of a nucleus containing combinations of neutrons and protons and one or more electrons bound to the nucleus by electrical attraction; the number of protons determines the identity of the element.
b.
an atom with one of the electrons replaced by some other particle: muonic atom; kaonic atom.
2.
Energy. this component as the source of nuclear energy.
3.
a hypothetical particle of matter so minute as to admit of no division.
4.
anything extremely small; a minute quantity.

Origin:
1350–1400; Middle English attomos, athomus < Latin atomus < Greek átomos, noun use of átomos undivided, equivalent to a- a-6 + tomós divided, verbid of témnein to cut


4. shred, speck, scintilla, iota, jot, whit.
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World English Dictionary
atom (ˈætəm)
 
n
1.  a.  the smallest quantity of an element that can take part in a chemical reaction
 b.  See also atomic structure this entity as a source of nuclear energy: the power of the atom
2.  any entity regarded as the indivisible building block of a theory
3.  See also atomism the hypothetical indivisible particle of matter postulated by certain ancient philosophers as the fundamental constituent of matter
4.  a very small amount or quantity; minute fragment: to smash something to atoms; there is not an atom of truth in his allegations
 
[C16: via Old French and Latin, from Greek atomos (n), from atomos (adj) that cannot be divided, from a-1 + temnein to cut]

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Etymonline
Word Origin & History

atom
late 15c., as a hypothetical body, the building block of the universe, from L. atomus (especially in Lucretius), from Gk. atomos "uncut," from a- "not" + tomos "a cutting," from temnein "to cut" (see tome). An ancient term of philosophical speculation (in Leucippus, Democritus),
revived 1805 by British chemist Dalton. Atom bomb is from 1945 as both a noun and a verb; cf. atomic.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

atom at·om (āt'əm)
n.

  1. A unit of matter, the smallest unit of an element, having all the characteristics of that element and consisting of a dense, central, positively charged nucleus surrounded by a system of electrons. The entire structure has an approximate diameter of 10-8 centimeter and characteristically remains undivided in chemical reactions except for limited removal, transfer, or exchange of certain electrons.

  2. This unit regarded as a source of nuclear energy.

  3. A part or particle considered to be an irreducible constituent of a specified system.

  4. The irreducible, indestructible material unit postulated by ancient atomism.

  5. An extremely small part, quantity, or amount.


a·tom'ic (ə-tŏm'ĭk) adj.
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American Heritage
Science Dictionary
atom  [%PREMIUM_LINK%]     (āt'əm)  Pronunciation Key 
The smallest unit of an element, consisting of at least one proton and (for all elements except hydrogen) one or more neutrons in a dense central nucleus, surrounded by one or more shells of electrons. In electrically neutral atoms, the number of protons equals the number of electrons. Atoms remain intact in chemical reactions except for the removal, transfer, or exchange of certain electrons. Compare compound. See also ion, isotope, orbital.

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American Heritage
Cultural Dictionary

atom definition


A unit of matter; the smallest unit of a chemical element. Each atom consists of a nucleus, which has a positive charge, and a set of electrons that move around the nucleus. (See Bohr atom.)

Note: Atoms link together to form molecules.
The American Heritage® New Dictionary of Cultural Literacy, Third Edition
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Published by Houghton Mifflin Company. All rights reserved.
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Example sentences
Carbon is the only atom that easily forms long chains of proteins that
  eventually make up life forms.
Each copper atom is surrounded by eight zincs and each zinc by eight coppers.
Once you catch an atom, you can do quite a lot with it.
Each kind of matter, such as paper, is made of atoms.
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