A compound is a substance formed when two or more different chemical elements bond together in a fixed ratio. Water is the classic example: two hydrogen atoms bonded to one oxygen atom, always in that exact proportion. This fixed ratio is what makes a compound fundamentally different from a simple mixture, where ingredients can be combined in any amount. The CAS Registry, the world’s largest database of known chemical substances, currently lists over 275 million unique chemicals, and the vast majority are compounds.
How Compounds Differ From Mixtures and Elements
An element is a pure substance made of only one type of atom. Gold, oxygen, and carbon are all elements. A compound combines two or more of these elements through a chemical reaction, creating something entirely new. Water doesn’t behave like hydrogen gas or oxygen gas on their own. Hydrogen is explosive, oxygen feeds fire, yet water puts fires out. When elements form a compound, the result has its own distinct set of properties.
A mixture, on the other hand, is just substances sitting together without chemically reacting. Trail mix is a mixture. Saltwater is a mixture. You can physically separate the parts of a mixture (filter, evaporate, pick out the pieces), but the only way to break a compound back into its individual elements is to destroy it through a chemical process like applying heat or running an electrical current through it.
Compounds also have a constant composition. Every molecule of table salt contains one sodium atom and one chlorine atom. Every molecule of water contains two hydrogen atoms and one oxygen atom. Mixtures have no such rule. You can make saltwater with a pinch of salt or a cup of it.
What Holds a Compound Together
The atoms in a compound are held together by chemical bonds, and the type of bond determines many of the compound’s properties. There are two main types.
Ionic bonds form when one atom transfers electrons to another. This typically happens between a metal and a nonmetal. Sodium gives an electron to chlorine, creating two oppositely charged particles (ions) that attract each other. The result is table salt. Ionic compounds tend to form solid crystals at room temperature and dissolve in water. Rather than existing as individual paired molecules, they arrange themselves in a repeating lattice structure, a three-dimensional grid where each positively charged ion is surrounded by negatively charged ions and vice versa.
Covalent bonds form when atoms share electrons instead of transferring them. This usually happens between two nonmetals. In water, oxygen shares electrons with two hydrogen atoms. Covalent compounds often exist as individual molecules, which is why many of them are liquids or gases at room temperature. Some covalent compounds, though, form continuous networks rather than individual molecules. Silica, the main component of sand, is a covalent compound where silicon and oxygen atoms bond in an enormous repeating network. A single grain of sand is essentially one giant molecule.
Organic vs. Inorganic Compounds
Scientists split compounds into two broad categories. Organic compounds have a backbone made of carbon atoms, usually bonded to hydrogen and often to oxygen, nitrogen, or other elements. Sugars, fats, proteins, DNA, gasoline, plastics, and most medications are organic compounds. The molecule that makes up table sugar (sucrose) contains 12 carbon atoms, 22 hydrogen atoms, and 11 oxygen atoms all bonded together.
Inorganic compounds generally lack that carbon-hydrogen framework. Water, table salt, baking soda, and rust are all inorganic. The line between the two categories isn’t perfectly clean (carbon dioxide is usually classified as inorganic despite containing carbon), but the distinction is useful because organic and inorganic compounds tend to behave very differently. Organic compounds, for instance, are more likely to burn, melt at lower temperatures, and dissolve in oils rather than water.
Compounds You Already Know
You encounter dozens of compounds every day without thinking about it:
- Water (H₂O) is the most familiar compound on Earth, essential to every living organism.
- Baking soda (NaHCO₃) is sodium bicarbonate, used in baking powder, antacids, and cleaning products.
- Table sugar (C₁₂H₂₂O₁₁) is sucrose, an organic compound derived from sugarcane or sugar beets.
- Calcium chloride (CaCl₂) is the “salt” spread on icy roads in winter to lower the freezing point of water.
- Nitrous oxide (N₂O) serves as a mild anesthetic in dental offices and as the propellant in cans of whipped cream.
Each of these has properties completely different from the raw elements it contains. Sodium is a metal that reacts violently with water. Chlorine is a toxic yellow-green gas. Together, they form table salt, something you sprinkle on food without a second thought. This transformation is the defining feature of a compound: the chemical reaction creates a new substance with its own characteristics.
How Compounds Are Named
Chemists follow standardized naming rules so that every compound has a clear, unambiguous name. For simple ionic compounds (a metal bonded to a nonmetal), the metal’s name comes first, followed by the nonmetal’s name with its ending changed to “-ide.” Sodium and chlorine become sodium chloride. Calcium and oxygen become calcium oxide.
Covalent compounds between two nonmetals use Greek prefixes to indicate how many atoms of each element are present. One nitrogen atom bonded to two oxygen atoms is nitrogen dioxide. Two nitrogen atoms bonded to one oxygen atom is dinitrogen monoxide (nitrous oxide). These naming conventions let scientists communicate precisely about millions of different substances without confusion.
Why Compounds Matter
Nearly everything around you is made of compounds. Your body runs on them: hemoglobin carries oxygen in your blood, glucose fuels your cells, and calcium phosphate gives your bones their strength. The medicines you take, the food you eat, the materials your house is built from, and the fuel that powers your car are all compounds or mixtures of compounds. Understanding what a compound is gives you the foundation to make sense of nutrition labels, medication ingredients, environmental news, and basic chemistry alike.