Molecules are formed by bonding atoms together. Can molecules be changed to atoms? Of course. All one has to do is break the chemical bonds.
If one breaks the bond with heat, the process is called pyrolysis. If a molecule is broken into fragments by reaction with water, the process is called hydrolysis. There are many chemical reactions that break molecules into smaller fragments. In some cases, there are individual atoms among the fragments.
Reduction reactions are specifically designed to produce pure metals composed of atoms. For example, most metalic ores are the oxides or sulfides of the metal. The industrial process of refining a metalic ore into the base metal, includes a chemical reaction known as reduction. In the refining of iron ore, carbon monoxide is the reducing agent. Iron ore containing ferric oxide is reduced in a blast furnace. The carbon monoxide steals the oxygen from the ferric oxide leaving iron atoms.
This is only one example of a chemical reaction that produces atoms from molecules. A more common example is photosynthesis. Plants take on water from their roots and absorb carbon dioxide from the air. In the presence of light, a chemical reaction takes place in the chloroplast of a living plant cell. The purpose of photosynthesis is to form glucose, the food the plant uses to grow, but a byproduct of the reaction is oxygen. Oxygen is an element that exists as atoms.
From a theoretical perspective, all one has to do to break an atom off from a molecule is to inject enough energy into the molecule to destroy the molecular bond. The determining factor is the strength of the bond holding one atom to another. Thus the bonds are so weak in some molecules that the molecule spontaneously degrades while other molecules have such strong bonds that large amounts of energy are required.
All molecules are made of atoms. Thus with the right conditions and the right amount of energy any molecule can be broken into atoms. Some atoms are stable enough to remain as elements for a long time. That is why elemental gold can be found in gold mines eons after the gold is formed in the magma of the Earth.
Other atoms are so reactive that once formed into atoms, they will react almost immediately with the surroundings, especially with air or water. That is why no miner has ever found a mother lode of metalic sodium.
It is the nature of chemical reactions that atoms are forever combining to form molecules while molecules are continually breaking down into atoms.
