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formula unit vs. molecule

formula unit

noun
  1. (of an ionic compound that does not form molecules, as most salts) the chemical formula with the least number of elements out of the set of empirical formulas having the same proportion of ions as elements: NaCl is the formula unit for the ionic compound sodium chloride.

molecule

[ mol-uh-kyool ]
noun
  1. the smallest physical unit of an element or compound, consisting of one or more like atoms in an element and two or more different atoms in a compound.
  2. a quantity of a substance, the weight of which, measured in any chosen unit, is numerically equal to the molecular weight; gram molecule.
  3. any very small particle.