Why can't you add single atoms of what you need to either side of the arrow?

In a chemical reaction, you cannot simply add single atoms to either side of the arrow because that would violate the law of conservation of mass, which states that matter cannot be created or destroyed in a closed system. Each reactant and product must be balanced, meaning the number of each type of atom must be the same on both sides of the equation. Adding atoms arbitrarily would disrupt this balance and misrepresent the actual chemical process occurring.