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A molecule is a group of two or more atoms held together by covalent bonds as a discrete unit. A compound is any substance made of two or more different elements in a fixed ratio. The two overlap but are not the same: molecular compounds like water (H2O) and methane (CH4) are both molecules and compounds, ionic compounds like table salt (NaCl) are compounds but exist as crystal lattices, not discrete molecules, and elemental molecules like H2 or O3 are molecules but not compounds, because they contain only one element.
If you close your eyes and think back to high school science, you can probably still remember your teacher incessantly discussing molecules and compounds in your chemistry class. It may not be a pleasant memory, and you may not remember what she was trying to explain, but molecules form the entirety of our existence, so a brief review of the jargon surrounding molecules and compounds is probably a good thing.
When you see a molecular diagram or read a molecular formula, it may not immediately be apparent whether you are seeing a molecule or a compound. In fact, you may not even know if there is a difference between these two designations at all! A popular textbook shortcut says “all compounds are molecules, but not all molecules are compounds.” That is a useful first approximation, but it gets ionic compounds like table salt wrong: NaCl is a compound but isn’t a discrete molecule, it’s a giant crystal lattice. Keep reading for the more careful version.
Molecules Vs Compounds
The words “molecule” and “compound” get thrown around very casually, often interchangeably, in any conversation about chemistry or material sciences. There is a good reason for this—the formal definitions are very nuanced, and there is some overlap, as well as debate about the boundaries of each classification.
Molecules
Simply put, molecules are particles composed of two or more atoms that are chemically held together; these molecules have a specific number of atomic nuclei, and can be considered an individual particle. Whether a particle consists of only one type of atom or a variety of different atoms, it is still considered a molecule. There are small molecules and large molecules, ranging from the most basic H2 molecule, containing only two hydrogen atoms, to a complex DNA macromolecule, which consists of millions of base pairs and atoms, “molecule” is a word that applies to a huge variety of particles.
However, if you were to look at a grain of salt, made up purely of NaCl, you wouldn’t consider it a molecule, as it is in fact a huge lattice network of individual sodium chloride compound particles, without a clear number of atomic nuclei. The same thing is true if you look at a drop of water; you aren’t looking at a molecule of water, but rather a huge grouping of liquid H2O molecules in a high enough concentration to be seen by the naked eye!
Now, in the past, molecules have been defined as those particles that were solely bonded through covalent bonds, meaning bonding that occurs through the sharing of electrons. In the case of O3, for example, an ozone molecule, the three oxygen atoms that compose that molecule are bonded in a unique way. Two of the oxygen atoms are bonded by a double covalent bond (two pairs of electrons being shared), and one of those oxygen atoms is simultaneously sharing a coordinate covalent bond with a third oxygen atom.
This type of classification made a clear distinction between other particles that used ionic bonding, but it was soon realized that real-world bonding sits on a continuous covalent-to-ionic spectrum, so classifying things as molecules purely by bond type is shaky. The IUPAC definition that most chemists use today calls a molecule "an electrically neutral entity consisting of more than one atom," which is still a discrete, countable particle, not a continuous lattice. Under that definition, ionic compounds (table salt, calcium carbonate) are compounds but not molecules, while elemental molecules like H2, O2 and O3 are molecules but not compounds. The intersection, things that are both, is what chemists call molecular compounds.
Covalent Compounds
This is a label that gets used to specifically denote when a molecule is composed of at least two different elements. Methane, for example, is a greenhouse gas that has received a great deal of attention in recent years. Methane has the chemical formula CH4, and consists of one carbon atom and four hydrogen atoms that are all covalently bonded to one another. Each hydrogen atom shares one electron with the carbon atom. Methane (CH4), water (H2O), carbon chloride (CCl4), ammonia (NH3) and countless other covalent compounds are considered both molecules and compounds, since they only rely on molecular bonds to form.
Ionic Compounds
Ionic compounds are similar to covalent bonds, but differ in terms of how the electrons of the two atoms interact. Ionic compounds a formally defined as a grouping of atoms of more than one element connected through ionic bonds. In a covalent (molecular) bond, the electrons are shared, whereas in an ionic bond, one or more electrons is given fully to another atom. This is because an ionic bond forms between oppositely charged ions; in simple terms, an ion may have a positive or negative charge, due to the excess or lack of an electron. When these oppositely charged ions connect, an ionic bond is formed. Whether ionic bonds are inherently "stronger" than covalent bonds depends on what you mean: individual ionic interactions in the gas phase can be moderate, but ionic crystals are held together by enormous cumulative lattice energies (hundreds to thousands of kJ/mol), which is why salt has such a high melting point. Strong covalent bonds in molecules can rival or exceed these (a C-C single bond is around 347 kJ/mol; a C=O double bond around 745 kJ/mol; a triple bond in nitrogen gas a remarkable 945 kJ/mol).
Perhaps the most common example of an ionic compound is table salt, NaCl. A neutral sodium atom (Na) has no charge, but if that atom were to lose an electron, it would become a sodium ion with a net charge of +1. Similarly, a neutral Chlorine atom has no charge, but if that atom were to gain an electron, it would have a net ionic charge of -1. When these two ions—a cation and an anion, respectively—come together, the extra electron from chlorine can be given to sodium. This stabilizes the two atoms, the outer electron shells of both atoms are satisfactorily filled, and an ionic compound is formed, boasting a strong bond that will be more difficult to break than a covalent bond.
A Final Word
Words have a great deal of power, so using the correct ones is important when discussing certain areas of science. In terms of molecules and compounds, there are some nuanced differences between the two, but for most purposes and conversations, the two can be used interchangeably. However, if you are discussing a molecule that consists of only one type of element, such as H2 or O3, be sure not to call it a compound! And next time you sprinkle some salt on a bland meal, offer up some thanks to ionic bonding!
