Molecule:
A molecule is the smallest unit of a substance that retains its chemical properties and consists of two or more atoms held together by chemical bonds. It is the fundamental building block of all matter. Molecules can be composed of atoms of the same element or different elements. The atoms in a molecule are chemically bonded together, which means they share electrons to form stable structures.

Compound:
A compound, on the other hand, is a substance composed of two or more different elements chemically combined in fixed proportions. It is formed when atoms of different elements chemically react with each other and form chemical bonds. Compounds have unique and distinct properties compared to their constituent elements. For example, water (H2O) is a compound made up of two hydrogen atoms and one oxygen atom. It has different properties than hydrogen gas (H2) or oxygen gas (O2).

Differences:

Composition:
- A molecule can be composed of atoms of the same element (e.g., O2, N2) or different elements (e.g., H2O, CO2).
- A compound is always composed of atoms of different elements (e.g., H2O, CO2).

Formation:
- Molecules can be formed through the combination of atoms by sharing electrons (covalent bonding) or by transferring electrons (ionic bonding).
- Compounds are formed through chemical reactions between different elements, where atoms are rearranged and form new chemical bonds.

Properties:
- Molecules have their own unique properties based on the types and arrangement of atoms within them.
- Compounds have properties that are distinct from the properties of their constituent elements. For example, sodium (a reactive metal) and chlorine (a toxic gas) combine to form sodium chloride (table salt), which is a stable compound with completely different properties.

Representation:
- Molecules can be represented using molecular formulas (e.g., H2O, CO2) or structural formulas.
- Compounds are represented using chemical formulas or names, indicating the types and ratios of elements present (e.g., NaCl, C6H12O6).

Examples:
- Examples of molecules include diatomic gases like oxygen (O2) and nitrogen (N2).
- Examples of compounds include water (H2O), carbon dioxide (CO2), sodium chloride (NaCl), and glucose (C6H12O6).

In summary, a molecule is the smallest unit of a substance that retains its chemical properties, while a compound is a substance composed of different elements chemically combined. Molecules can be composed of the same or different elements, while compounds are always composed of different elements. Molecules and compounds have different formation processes, properties, and representations.