When writing a chemical equation, the number of atoms on the reactants do NOT need to equal the number of atoms on the products. true
false

False

Explanation:

When writing the chemical equation number of atoms on the reactant side must be equal to the number of atoms on right side because according to law of conservation of mass, mass can neither be created nor destroyed but change from one to another form, thus total mass remain same on both side of equation.

Law of conservation of mass:

According to the law of conservation mass, mass can neither be created nor destroyed in a chemical equation.

This law was given by French chemist  Antoine Lavoisier in 1789. According to this law mass of reactant and mass of product must be equal, because masses are not created or destroyed in a chemical reaction.

For example:

In given photosynthesis reaction:

6CO₂ + 6H₂O + energy → C₆H₁₂O₆ + 6O₂

there are six carbon atoms, eighteen oxygen atoms and twelve hydrogen atoms on the both side of equation so this reaction followed the law of conservation of mass.