The water molecule is made up of two hydrogen atoms and one oxygen. Already the nitrogen gas, which exists in the atmosphere, is formed by the union of two nitrogen atoms. The same goes for oxygen gas.
We then say that oxygen is a simple substance, since it is formed by the union of chemically equal atoms. Water is a compound substance, because it is formed by the union of different atoms.
Understand better by analyzing the following image:
Some compound substances are formed by different ions, such as sodium chloride, which has sodium and chlorine ions.
Therefore, a pure substance can be simple when it is formed by only one type of atom, or compound when there is more than one type of atom or ion in its formula.
Simple Substances are those formed by a single type of chemical element.
Examples: H2, O2, O3, Cl2, P4 .
Composite substances are those formed by more than one type of chemical element.
Examples: NaCl, H2The CA2ONLY4, HCl, H3POWDER4.