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.
Molecules
A molecule is a group of two or more atoms covalently bonded together. Some example molecules are listed in the table below.
Name | Chemical formula | Lewis structure | Ball-and-stick model |
---|---|---|---|
Molecular oxygen | |||
Water | |||
Carbon dioxide | |||
Methane | |||
Ammonia |
Samples of molecular substances consist of lots of individual molecules. For example, an oxygen tank is full of individual
Diatomic elements
Some elements do not exist alone in nature as individual atoms. These diatomic elements are found as a covalently-bonded pairs, like molecular oxygen
Other common diatomic elements include
Diatomic elements are considered molecules, but not compounds. A compound must have two different elements bonded together.
Covalent network solids
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
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
So, a grain of sand is essentially one giant molecule! But typically, chemists do NOT call network solids like silica "molecules."
Salts
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
A
Notice that unlike covalent molecules and network solids, there aren’t any bond lines (“sticks”) between the ions. That’s because covalent bonds involve shared pairs between two specific atoms, but ionic bonds involve one ion attracting several surrounding ions. So, we can’t place the bonds in specific spots for salts.
Compounds
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,
However, not all chemical substances are compounds. For example, helium gas is composed solely of individual
Similarly, oxygen gas
Name | Chemical formula | Molecule? | Salt? | Compound? |
---|---|---|---|---|
Helium gas | no | no | no | |
Oxygen gas | yes | no | no | |
Water | yes | no | yes | |
Sodium chloride | no | yes | yes |
Chemicals
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.
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.