The general meaning of "atomic" is "irreducible". That is, reduced to the smallest possible part. It has more specific meanings in several contexts:

• In chemistry, chemical atoms are grouped by type in the periodic table and consist of particles as described by particle physics.
• In computer science, an atomic operation is one that cannot be subdivided. Or more specifically, an operation that always completes as a whole, or doesn't complete at all. See atomic (computer science).

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