The atomic theory of matter seeks to explain the nature of matter-the materials of which the Universe, all galaxies, solar systems and Earth are formed.

First postulated by JOHN DALTON, the atomic theory of matter contends:

Each chemical element is made of fundamental units called ATOMS.

All of the atoms of a given element are identical.  Gold, copper, iron, zinc-these are all individual elements made up of the same atoms.  All gold atoms are exactly like all other gold atoms and unlike any other kind of atom. All copper atoms are exactly like all other copper atoms and unlike any other kind of atom.

Atoms combine with one another to form more complex units called molecules.  All molecules are made of constituent atoms.  All molecules of water are made of two atoms of hydrogen and one atom of oxygen-no other element ever makes water.  All water molecules are exactly like all other water molecules-they are all made of only hydrogen and oxygen in the atomic ratio of two to one.

ATOMS ARE FUNDAMENTAL UNITS OF MATTER AND ALL ATOMS ARE CONSERVED IN CHEMICAL REACTIONS.
IN CHEMICAL REACTIONS, ATOMS ARE REARRANGED AND REGROUPED TO FORM COMPOUNDS  (NAME WE GIVE TO MOLECULES FORMED FROM ATOMS).  FOR EXAMPLE, TWO ATOMS OF HYDROGEN AND AN ATOM OF OXYGEN REACT TO FORM THE COMPOUND-WATER.  THIS COMPOUND IS MADE OF MOLECULES OF WATER.

LAW OF CONSERVATION OF MATTER

   THIS SCIENTIFIC LAW TELLS US THAT ALL ATOMS ARE CONSERVED IN A CHEMICAL REACTION.  YOU END UP WITH THE SAME MASS AS YOU BEGAN.
FOR EXAMPLE, WHEN TWO ATOMS OF HYDROGEN REACT WITH ONE ATOM OF OXYGEN, EXACTLY ONE MOLECULE (MADE UP OF THREE ATOMS)  OF WATER IS PRODUCED.  THERE IS ABSOLUTELY NO LOSS OF MASS AS A RESULT OF THIS CHEMICAL OR ANY CHEMICAL REACTION.

LAW OF CONSTANT COMPOSITION
   THIS SCIENTIFIC LAW TELLS US THAT ATOMS REACT IN PREDICTABLE WAYS THAT RESULT IN COMPOUNDS OF CONSTANT (ALWAYS THE SAME) COMPOSITION.
THE MOLECULE AMMONIA HAS THE FORMULA: NH₃.  THIS MEANS THAT ONE ATOM OF NITROGEN ALWAYS COMBINES WITH THREE ATOMS OF HYDROGEN TO GIVE ONE MOLECULE OF AMMONIA.  THE COMPOSITION OF AMMONIA NEVER VARIES, IT IS CONSTANT-NO MATTER WHETHER IT OCCURS ON ANOTHER PLANET, ANOTHER GALAXY, NO MATTER WHERE.