Answer:
ATOMS;in the simplest way we can define an atom as the building blocks of all the matter in the universe.atoms are very small that cannot be compared to anything else
MOLECULES;a molecule is the smallest particle of an element or a compound that is capable of an independent existence and shows all the properties of that substance
ELEMENTS;elements are the basic form of a matter that cannot be broken down into simpler substances by chemical reaction,Elements are normally divided int metals nonmetals and metalloids.
COMPOUNDS;A compound is a substance composed of two or more elements,chemically combined with one another in a fixed proportion.