Why equal gas volumes can contain equal numbers of particles
A gas’s particles are far apart, so the space between them dominates its volume. At a fixed temperature and pressure, that volume is governed mainly by how many particles are moving around—not by whether they are light or heavy, or what they are made of. Heat the gas, squeeze it, or add more particles, and its volume changes; swap one kind of gas for another while holding temperature and pressure steady, and the same number of particles occupies the same volume, to a close approximation.
That is the physical idea behind Avogadro’s proposal. It applied to molecules, not necessarily individual atoms: a particle of a gas might be a single atom, or a small group of atoms joined together.
The distinction that made gas reactions make sense
Chemists already observed that gases combined in simple volume ratios. But those ratios were difficult to reconcile with the idea that every gas particle was a single atom. If an elemental gas could consist of paired atoms, its particles and its atoms were not the same thing. Counting equal-volume particles made room for that distinction and helped chemists connect measured gas volumes to chemical formulas and atomic weights.
The lasting payoff was a clearer way to read a reaction: a volume ratio counts molecules in the reacting gases, while each molecule may carry one or several atoms. The same-sized gas sample can therefore contain the same number of particles without containing the same number of atoms.
