Matter is neither created nor destroyed during the chemical reaction of photosynthesis. In keeping with the Law of Conservation of Matter, the mass of the products formed is equal to the mass of the reactants. Here's the equation for the reaction:
`6 CO_2 + 6 H_2O -> C_6H_(12)O_6 + 6 O_2`
You can see that every atom is accounted for. There are 6 carbon atoms, 12 hydrogen atoms and 18 oxygen atoms on each side of the equation.
Sometimes it's hard to see that the Law of Conservation of Mass is obeyed when gas is involved. Antoine Lavoisier, a French chemist of the 16th century, proposed the Law of Conservation of Mass and convinced his skeptical colleagues of its validity when he was able to prove that gas from the air was part of the reaction. He designed experiments in a closed system in which he could weigh the air surrounding the reaction vessel. When he included the mass of the air, gaseous products and reactants were accounted for and his results showed no loss or gain of mass.