|a printed punctuation mark (‽), available only in some typefaces, designed to combine the question mark (?) and the exclamation point (!), indicating a mixture of query and interjection, as after a rhetorical question.|
|a gadget; dingus; thingumbob.|
|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 |
atom at·om (āt'əm)
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.
This unit regarded as a source of nuclear energy.
A part or particle considered to be an irreducible constituent of a specified system.
The irreducible, indestructible material unit postulated by ancient atomism.
An extremely small part, quantity, or amount.
|atom (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.