The number of atoms in a molecule, or the degree to which something is atomic in nature; the quality of being composed of atoms.
From atomic + -ity (suffix forming nouns from adjectives). Used in chemistry to describe the specific number of atoms in molecular structures.
Atomicity matters enormously—oxygen gas (O₂) has different properties than ozone (O₃) even though they're both oxygen, so counting atoms is like counting the ingredients in a recipe.
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