What do you mean by:
(a) molecule of an element
(b) molecule of a compound, with examples

Hint: We know that the atom is the basic unit of a chemical element. All kinds of matter in the universe is made up of different substances called elements. Elements, molecules, compounds and mixtures are different with each other, but similar to some aspects.

Complete step by step solution: They are the fundamental building blocks of element and matter. Atom is made up of three subatomic particles-protons, electrons and neutrons. For example, carbon is an atom of a chemical element. It is composed of six electrons and protons.
A substance that is made entirely from one type of atom is called an element. For example, a hydrogen element is made from atoms containing a single proton and electron. If the number of protons is changed, the element is changed.
When two or more atoms are combined together to form a chemical bond, then it is called a molecule. The atoms may be of the same element or of different elements.
(a) If atoms of the same element are combined to form a chemical bond, then it is called a molecule of an element. For example, ${{{H}}_2},{{{O}}_2},{{{N}}_2},$ etc.
(b) If atoms of different elements are combined to form a chemical bond, then it is called compound. For example, ${{C}}{{{O}}_2},{{{H}}_2}{{O}}$ etc.
Elements will be in a specific ration in a compound. Molecular formula tells the ratio of the elements in a compound.

Note: Single atoms of elements are not molecules. A single oxygen is not a molecule since it has a single atom of element. All compounds are molecules but all molecules are not compounds. The symbols of elements make up the molecular formula.