* Smallest "thing": The smallest unit of an element is an atom.

* Mixtures vs. Compounds:

* Mixtures are combinations of substances where each substance retains its own properties. There's no fixed ratio, and they can be separated.

* Compounds are formed when two or more elements chemically combine in a fixed ratio. They have properties different from the original elements.

* Size isn't the only factor: While we can talk about the relative size of atoms (hydrogen is the smallest), the "smallest" mixture or compound depends on what we're comparing:

* By mass: The smallest compound by mass would be one with the lowest molecular weight, like hydrogen gas (H2).

* By volume: The smallest mixture by volume would depend on the specific substances involved.

Therefore, there's no single "smallest" element, mixture, or compound. It depends on how you define "smallest".