Atom – all substances are made up of matter and the fundamental unit of matter is the atom. The atom constitutes the smallest particle of an element.
Element – An element is a substance made up of atoms of one kind.
Compound – A compound is formed when atoms or molecules of different elements combine. In a compound, elements are chemically combined in a fixed proportion.
Examples:-
Hydrogen and oxygen are combined in a fixed proportion of 2:1 to form the compound water.