The question of existence arises from all the materials including, a rock, a cow, a basic human heart; the basic building block of them is what amaze us. Of, course it is the atoms. They are the fabric of all things that exist. To understand that, we should have a proper understanding of law of conservation of mass.
If we consider an isolated system, that has a defined boundary and no energy source can cross that barrier. In this system, mass or matter cannot be created or destroyed. The Universe itself is an isolated system.
Now considering six carbon atoms, twelve hydrogen atoms and eighteen oxygen atoms These molecules can get moving with only a little amount of energy. They can bond together and form a familiar molecule. There’s water and carbon-di-oxide that forms. Taking six molecules of carbon di-oxide and water molecule and adding a bit of energy, they form a sugar compound.
6CO2 +6H20 à C6H1206
Therefore, no matter is lost. The applied energy has now changed into bonds that are there between the atoms. That energy can again be released if sugar breaks back into water and carbon di-oxide.
Considering another example,
Methane which is related to cow flatulence is also used as rocket fuel.
Adding oxygen and little quantity of energy, results in combustion and forming carbon dioxide, water and releasing energy.
CH4 + 2O2+ ENERGY à CO2 +2H2O+ ENERGY
Thus, again the atoms and molecules are balanced.
Taking one last example,
Propane is another combustible gas; Adding oxygen and lighting it up gives more water and CO2
C3H8+5O2+ ENERGY à 3CO2 + 4H2O
Similar adjustment of molecules in the overall reaction.
Thus, there are many examples proving the law of conservation of mass, holing its truth and verification.
Understanding From The Universe
Hydrogen was formed under 3minutes, from a high energy soup of particle, during the birth of the universe. Stars are formed from accumulation of clusters of atoms. Inside these stars, nuclear reactions take place by fusion of light elements such as hydrogen and helium to form carbon and oxygen. These reactions might give a confusion about breaking the law due to excessive release of energy. But from Einstein’s equation it is known that energy is equivalent to mass. The total mass of the starting atom is slightly more than the mass of the products. Thus, the loss of mass is what corresponds to the energy gained. This energy is radiated from the star as light and heat. Soon, this star becomes supernova and scatter itself all across the space. These atoms and atoms from supernovas later join together after billions of years they found each other in the earth, and starting acting in our little isolated system.