Molecules, salts, compounds, and chemicals: What's the difference?

Chemistry is the study of matter in all its forms. So, it’s no surprise that chemists need different terms to describe the various ways matter can be structured. Let’s take a closer look at what some of those terms mean.

A molecule is a group of two or more atoms covalently bonded together. Some example molecules are listed in the table below.

Samples of molecular substances consist of lots of individual molecules. For example, an oxygen tank is full of individual $\mathrm{O}{\phantom{A}}_2$‍ molecules, and a glass of water is full of individual $\mathrm{H}{\phantom{A}}_2\mathrm{O}$‍ molecules.

Though many covalently bonded substances consist of molecules, some do not. Instead, some covalent substances are bonded together in a network.

An example of a covalent network solid is silica $(\mathrm{SiO}{\phantom{A}}_2)$‍, the main component of most sand. An atomic model of silica is shown below, consisting of a network of silicon and oxygen atoms.

The chemical formula for silica tells us that it contains two oxygen atoms for every one silicon atom. But, the atoms are connected in a network rather than individual $\mathrm{SiO}{\phantom{A}}_2$‍ molecules.

So, a grain of sand is essentially one giant molecule! But typically, chemists do NOT call network solids like silica "molecules."

In chemistry, a salt is a substance composed of cations and anions ionically bonded to each other.

We've seen that covalently bonded substances can form molecules or network solids, but what about ionically bonded substances?

Ionic bonds form between oppositely charged ions. So, while you may think of a salt like sodium chloride $(\mathrm{NaCl})$‍ as being composed of individual sodium chloride “molecules,” it’s not.

A $\mathrm{Na}{\phantom{A}}^{+}$‍ ion attracts not just one $\mathrm{Cl}{\phantom{A}}^{-}$‍, but all the $\mathrm{Cl}{\phantom{A}}^{-}$‍ ions that surround it. This is true of all the ions in the solid. So, ionic solids arrange themselves in a repeating lattice structure, like the model $\mathrm{NaCl}$‍ lattice shown below.

Another common term in chemistry is "compound." A compound is a chemical substance composed of two or more different elements. Compounds can be covalently or ionically bonded.

For example, $\mathrm{H}{\phantom{A}}_2\mathrm{O}$‍ is a covalent compound because it contains hydrogen and oxygen (different elements) covalently bonded together. $\mathrm{SiO}{\phantom{A}}_2$‍ is also a covalent compound.

$\mathrm{NaCl}$‍ is an example of an ionic compound because it contains sodium and chlorine (different elements) ionically bonded together. In fact, all salts are compounds because only ions of opposite charge—and therefore, different elements—attract each other.

However, not all chemical substances are compounds. For example, helium gas is composed solely of individual $\mathrm{He}$‍ atoms, so it is neither a molecule nor a compound.

Similarly, oxygen gas $(\mathrm{O}{\phantom{A}}_2)$‍ is composed entirely of oxygen atoms. Each $\mathrm{O}{\phantom{A}}_2$‍ is a molecule, but not a compound.

Last but not least, let's talk about chemicals. A chemical (or chemical substance) is any substance with a definite composition. In other words, we can describe what elements the substance is made of, their ratio, and how they're arranged.

For example, water is a chemical because it always consists of one oxygen atom bonded to two hydrogen atoms. Every water molecule has the same composition, whether it's in the ocean, in a cloud, or on Mars.

All of the preceding categories—molecules, salts, and compounds—are chemicals. Even pure elemental substances which do not fall into any of those categories (e.g. $\mathrm{He}$‍ gas) are chemicals.

In some situations, you may see the word "chemical" used to refer exclusively to substances that are harmful or made in a laboratory. For example, it may sound bad if you hear that "chemicals" are mixed into your drinking water. This is indeed bad if the chemicals are toxic. From a chemistry perspective though, even pure water is a chemical.

Because "chemical" is such a broad term and can mean different things to different people, chemists are typically more specific when referring to substances. They use names and formulas to describe exactly what a substance is composed of.

So ironically, "chemical" is not the best word to use in chemistry! There are more descriptive options to choose from.