Then only certain molecules can exist. The total number of atoms that react in a chemical reaction does not change. The reaction occurs because the atoms rearrange themselves into new, different groups. Therefore, we say that the atoms are conserved in both their number and type. Matter can not be made nor destroyed.
All matter is constant.
In a chemical reaction, the amount of energy that is used in the initial reaction, remains the same once the process is complete. Energy is neither created, nor destroyed. Although sometimes energy is labeled as a product, this is only potential energy that is released. Mass and energy can be changed from one form to the other, but their sums are always constant. It can not increase or decrease.