An atom is the basic unit of an element. When you see the chemical formula for water, H2O, it's telling you that each molecule of water is made up of two atoms of hydrogen and one atom of oxygen.

In science class, you've probably come across atoms, the bits that make up molecules. Less scientifically, the word atom can also mean a very small piece of anything at all. The Greek root of atom is atomos, which means "indivisible," since the scientists who first gave the atom its name imagined it couldn't be split or divided into smaller pieces. Even though we know there are smaller things that make up an atom (protons, neutrons and electrons), it's still a good way to think about the word atom.

Definitions of atom
  1. noun
    (physics and chemistry) the smallest component of an element having the chemical properties of the element
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    one of two or more atoms with the same atomic number but with different numbers of neutrons
    (chemistry) an atom having a valence of one
    carbon atom
    an atom of carbon
    hydrogen atom
    an atom of hydrogen
    free radical, radical
    an atom or group of atoms with at least one unpaired electron; in the body it is usually an oxygen molecule that has lost an electron and will stabilize itself by stealing an electron from a nearby molecule
    a radioactive isotope of an element; produced either naturally or artificially
    acid hydrogen, acidic hydrogen
    a hydrogen atom in an acid that forms a positive ion when the acid dissociates
    deuterium, heavy hydrogen
    an isotope of hydrogen which has one neutron (as opposed to zero neutrons in hydrogen)
    type of:
    the real physical matter of which a person or thing consists
  2. noun
    (nontechnical usage) a tiny piece of anything
    synonyms: corpuscle, molecule, mote, particle, speck
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    a relatively small granular particle of a substance
    material resulting from the process of grinding
    a microscopic particle of triglycerides produced in the intestines during digestion; in the bloodstream they release their fatty acids into the blood
    a tiny dark speck made by the excrement of a fly
    identification particle
    a tiny particle of material that can be added to a product to indicate the source of manufacture
    a tiny grain
    (trademark) a microscopic and traceable identification particle used to trace explosives or other hazardous materials or to prevent counterfeiting
    type of:
    material, stuff
    the tangible substance that goes into the makeup of a physical object
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