Any units will do. for example, let's use the unit of "dozens":

2 dozen of h2 particles + _(1, 2, or dozen of o2 particles → _(1, 2, or 3)_ dozen water particles

2 dozen of H₂ particles  + 1 dozen of O₂ particles  → 2 dozen of H₂O particles.

Explanation:

2H₂ + O₂ → 2H₂O.

So, every 2.0 moles of H₂ react with 1.0 mole of O₂ to produce 2.0 moles of H₂O.

So, the represented reaction of the problem is:

2 dozen of H₂ particles  + 1 dozen of O₂ particles  → 2 dozen of H₂O particles.

< > according to newton's third law of motion, forces always act in equal but opposite pairs. another way of saying this is for every action, there is an equal but opposite reaction.< >