Surface-to-volume ratio
The fraction of a particle's atoms that sit on its surface — the mechanism behind most nanoscale behaviour.
Halve every edge of a cube and its volume falls eightfold while its surface area falls only fourfold. Repeat that enough times and surface atoms go from negligible to dominant: about 1% of a 100 nm particle, around 45% of a 3 nm one.
Since chemistry happens at surfaces, reactivity can shift by orders of magnitude with no change to composition at all.
Where this comes up
Articles that define surface-to-volume ratio in context.
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