At a chemical reaction no particles are lost or appear spontaneously from nothing;
during a chemical reaction, particles changes their character and/or there position.
The participating particles do regroup themselves.
You can approach this in another way: during chemical reactions, the particles are removing from oneanother, and after that, they approach each other again to connect in a new way.
In general we may say: reaction between particles only take place at the moment that those particles meet, have contact.
The particles must collide.
And then more: not every collision leads to reaction; that collision must take place in the very right way, must be effective and have sufficient energy.
The particles can be atoms, ions or molecules, and can be small or big.
att.: A macromolecule can react with a small ion in the mol-proportion 1:1
Make a nice estimation of the mass proportion of the reactants, in such a case.