The terms molecule and compound are often used interchangeably, but chemistry defines them distinctly. Understanding this difference is fundamental to grasping how matter is organized and how chemical reactions occur. Both concepts describe structures formed from atoms, but the key distinction lies in whether the bonded atoms are of the same element or different elements.
What Defines a Molecule
A molecule is the smallest identifiable unit of a pure substance that retains its characteristic composition and chemical properties. A molecule consists of two or more atoms chemically bonded together through strong forces, typically covalent bonds. These bonded atoms form a discrete, electrically neutral group with a specific, stable arrangement.
Crucially, a molecule can be formed from atoms of the same element. For instance, the oxygen we breathe exists as a diatomic molecule (O2), consisting of two oxygen atoms bonded together. Nitrogen gas (N2) and sulfur (S8) are also molecules, demonstrating that the identity of the bonded atoms does not need to change for a molecular structure to exist.
What Defines a Compound
A compound is defined as a substance formed when atoms of two or more different elements are chemically bonded together in a fixed ratio. This combination results in a pure substance with a uniform composition and chemical properties distinct from its constituent elements. For example, water (H2O) is a compound because it is formed from hydrogen and oxygen, two different elements, in a precise two-to-one atomic ratio.
Forming a compound involves a chemical reaction that creates strong chemical bonds, such as covalent or ionic bonds. Table salt, or sodium chloride (NaCl), is another common compound formed from sodium and chlorine. Compounds cannot be separated into simpler substances by physical means like filtration or evaporation.
The Overlap and Key Distinction
The relationship between molecules and compounds is a subset: all compounds are molecules, but not all molecules are compounds. This difference hinges entirely on the variety of elements involved. A molecule is the broader category, requiring two or more atoms to be chemically linked, regardless of whether those atoms are identical or different.
The simplest molecule, like O2, is not a compound because it is composed only of oxygen atoms. However, a molecule like carbon dioxide (CO2) is simultaneously classified as a compound because it contains two different elements, carbon and oxygen, chemically bonded together.
For substances like table salt (NaCl), which is an ionic compound, the term “molecule” is sometimes avoided in favor of describing its structure as a crystal lattice of ions. In the case of covalent substances like water, the individual H2O unit is both a molecule and a compound, clearly illustrating the overlap. The defining difference remains that a compound must have atoms from at least two different elements.