Compound Facts

Compounds ['käm?poundz]

A thing composed of two or more separate elements.

Atoms and Elements: The Building Blocks of Everything

Photo of teacher and student doing chemistry experiments

Everything that takes up space in our world is matter. The science of studying matter is called chemistry. All matter is made of tiny particles called atoms. Atoms make up the air, the water, your body, your clothes, the food you eat, and the chair you are sitting on. But you can't see atoms. They are so tiny that there are billions of atoms just in the period at the end of this sentence. There are different types of atoms, and they fit together in special ways to make up all of the matter in the world.

All atoms are made up of even tinier particles called protons, neutrons, and electrons. The protons and neutrons huddle together in the nucleus, or center, of the atom, while the electrons spin around the nucleus.

Everything in the universe is made from elements. We know of 92 elements that occur in nature, but several more have been created by scientists for a total of 118. An element is a pure substance that is made from one single type of atom. For example, gold is an element that is made up of gold atoms. Oxygen is an element made up of oxygen atoms. The Periodic Table of Elements is used to organize the elements by their atomic number, or the number of protons in the nucleus. For example, oxygen has 8 protons, so its atomic number is 8. Chemical symbols stand for each element. There is much more you can learn about atoms and elements from the periodic table.

Periodic Table of the Elements


When two or more atoms join together, we call it a molecule. When two or more atoms of different elements join together, we call it a compound. All compounds are molecules, but not all molecules are compounds. That is because a molecule can be made up of two atoms of the same kind, as when two oxygen atoms bind together to make an oxygen molecule. However, all compounds are made up of two or more different types of atoms.

Diagram of molecules compared to compounds

Elements are rarely found in their pure state; compounds are much more common. There are just over 100 different kinds of atoms, but there are millions of different kinds of substances made up of different types of molecules. Probably everything you see around you is some type of compound. When atoms of different kinds combine to form a compound, a new substance is created. New compounds do not have the same physical or chemical traits of the original elements. They have a new life of their own.

Compounds are written with formulas showing which elements from the periodic table are combined. One very familiar compound is water. When two hydrogen atoms (H2) combine with one oxygen atom (O), it makes the compound H2O, which we know as water. All water molecules have this same combination of atoms. Water is not hydrogen or oxygen. You couldn't pour oxygen and hydrogen atoms on a fire and expect to put it out. But when they are bonded together as water molecules, they behave like water. A compound is a brand-new substance with its own properties.

Image of common names to compound names and chemical formula

The same elements can build very different compounds. If you took those two hydrogen atoms and joined them to two oxygen atoms (instead of one), you would wind up not with water but with H2O2, a very different compound called hydrogen peroxide - you wouldn't want to drink it!

There are many other compounds that are already familiar to you:

  • When one sodium atom (Na) combines with one chlorine atom (Cl), it makes the compound NaCl, which we know as salt.
  • Every time you breathe out, your breath contains CO2, a compound of one carbon atom (C) and two oxygen atoms (O2) that we call carbon dioxide.
  • Sometimes more than two elements make up a compound. A sugar molecule (glucose) is a compound of 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms, written as C6H12O6. These specific atoms in these exact numbers make up a sugar molecule.
  • When four different kinds of atoms (sodium, hydrogen, carbon, and oxygen) are combined in a certain way, we get NaHCO3, which you know as baking soda. If these atoms were combined in a different way, it would not be baking soda.

Chemical Bonds

How do these compounds form? What holds the atoms in a molecule together? The answer is that compounds are formed when elements are joined and held together by strong forces called chemical bonds. These bonds involve the electrons that orbit the nucleus of the atom. Electrons are located in energy levels that occur at certain distances from the nucleus, called shells. These shells can each carry a certain number of electrons (for example, 2 in the first shell, 8 in the second, and so on.) Atoms want to have their shells full with as many electrons as they can carry, and when their outermost shell isn't full, atoms try to bond with other atoms by giving up or gaining electrons. Atoms with an almost-empty outer shell will want to give up electrons, while atoms with an almost-full outer shell will want to gain electrons in order to fill it up.

Diagram of chemical bonds

There are two main types of bonds that hold most compounds together. Ionic bonds form when one atom gives up or donates, an electron to another in order that both will have a full outer shell. In doing so, the atoms are bonded and create a compound. Covalent bonds share electrons between atoms in order to fill their electron shells. In the compound, molecules are held together by the attraction between the nucleus and the shared electrons.

Diagram of ionic and covalent bonds

Compounds and Mixtures

Often substances may combine without forming a compound. To make a compound, there must be a chemical reaction where bonds are formed and an entirely new substance is created. Without that chemical reaction, combined substances may instead form a mixture.

Diagram of mixtures vs compounds

The components of a mixture keep their original properties and can easily be separated. For example, a mixture of fruits in a salad can be separated back into groups of different kinds of fruit. Salt and water can be combined in a mixture, but water is still water, and salt is still salt. To separate the two components, the water can be evaporated so that the salt can be collected. Sand and water can be separated by using a filter. The ocean, rocks, blood, and even the air we breathe are mixtures rather than compounds.

On the other hand, the components in a compound cannot be separated by physical means. Learn more about compounds and mixtures.

Physical and Chemical Changes

Physical changes do not break down compounds. Physical changes affect the size, shape, or state of the substance, but not the chemical properties. You can change the state of matter, but the compound does not change. If you leave an ice cube out in the sun it will melt into liquid water, but in either state, it is still made of water molecules. You can apply a physical force to a solid glass and break it, but the molecules that makeup glass will remain.

Diagram of physical verses chemical change

Chemical changes in compounds happen when chemical bonds are created or destroyed. Then the molecular structure changes; new molecules form and a new substance is created. Often heat is used to begin a chemical change, as when baking a cake. Another example of a chemical reaction is the rusting of a metal trash can. The rusting happens because the iron (Fe) in the metal combines with oxygen (O2) in the air. Chemical bonds are created and destroyed to eventually make iron oxide (Fe2O3), which we call rust.

It is not easy to break chemical bonds, but it can be done in chemical reactions using energy to break the bonds. For example, an electric current passed through water can cause a chemical change that breaks water down into hydrogen and oxygen. When a chemist mixes different compounds in a chemical reaction, the compounds may join together to make one compound or change into several new compounds. Some of the signs of a chemical reaction are a change in temperature, the formation of a gas, or a color change.

Naming Compounds

Scientists have a specific way of naming compounds. There are some complex rules, but let's focus on the simple ones. For molecules with two elements, the compound name has two words: the name of the first element, and the name of the second element changing its ending to "ide." For example, if oxygen is the second element in the compound, it become "oxide." If chlorine is the second element, it becomes "chloride."

If one of the elements has more than one atom, you add a prefix to the beginning of the name of the element, depending on the number of atoms. If there are two atoms, you add "di" at the beginning. If there are three, you add "tri" at the beginning. If there are four, you add "tetra."

The compound of one atom of sodium and one atom of chlorine is named sodium chloride.
(1) Sodium and (1) Chlorine = Sodium Chloride (NaCl)

The compound of one atom of magnesium and one atom of sulfur (MgS) is named magnesium sulfide.
(1) Magnesium and (1) Sulfur = Magnesium sulfide (MgS)

The compound of one atom of carbon and two atoms of oxygen is named carbon dioxide.
(1) Carbon + (2) Oxygen = Carbon dioxide (CO2)

The compound of one atom of carbon and four atoms of chlorine is named carbon tetrachloride. (1) Carbon + (4) Chlorine = Carbon tetrachloride (CCl4)

With a little practice, you'll soon be speaking the "language" of chemical compounds!