Protons, it was once organized by atomic mass but organizing by protons turned out to be better
The order of placement of the elemnts on the modern Periodic Table is determinated by:
1)atomic number (Z=number of protons).
If the mass of all of the products in a chemical reaction is equal to 100g then the mass of the reactants in that same reaction had to have had a mass of 100g this is due to the law of conservation of matter stating matter cannot be created or destroyed in a chemical reaction.
