atom

atom


at·om    Audio Help   (āt'əm)   

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

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

    3. 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.

    4. This unit regarded as a source of nuclear energy. See Table at subatomic particle.

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

  2. Physics & Chemistry

    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. See Table at subatomic particle.


[Middle English attome, from Latin atomus, from Greek atomos, indivisible, atom : a-, not; see a-1 + tomos, cutting (from temnein, to cut; see tem- in Indo-European roots).]
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