Atoms make up all the objects and substances in our world. But they rarely exist as single atoms, alone or unattached. They are usually attached or joined to other atoms. For example, oxygen, a gaseous element that makes up one fifth of the air on Earth, does not float about as single atoms of oxygen, O (its chemical symbol). It is in the form of oxygen atoms joined together in pairs, written as O₂. Two or more atoms linked or joined together make a molecule. O₂ is a molecule of oxygen. Atoms of different elements often link together as molecules, too. These molecules are known as compounds.

Compounds

If atoms of one chemical element join or combine with atoms from other elements, in a process called a chemical reaction, a compound is formed. O₂ is a molecule of oxygen but not a compound: the two atoms are of the same element. Two atoms of hydrogen and one of oxygen form a very familiar substance: water, written as H₂O. This is both a molecule and a compound.

Some compounds, like minerals in rocks, may have, say, between 50 and 100 atoms from 15 or 20 different elements in each molecule. Other compounds, like certain plastics, have millions of atoms in each molecule, but these are usually from only a few elements—mainly carbon, hydrogen, oxygen and nitrogen.

Chemical formulae

To describe chemicals, chemists use formulae. All chemical elements are given one- or two-letter symbols. The formula for a compound shows which elements it contains and in what proportions. All substances have a chemical name—as well as a common name if the substance is well known—revealing what its chemical composition is. For example, the chemical name for chalk is calcium carbonate. Its chemical formula is CaCO₃, meaning that every molecule has one calcium atom, one carbon atom and three oxygen atoms.

Bonds

The links between atoms are called bonds. There are different types of bonds depending on the atom’s structure and the conditions such as temperature and pressure. In a covalent bond one atom shares an outer electron with another atom. Two hydrogen atoms are bonded covalently to one oxygen atom to form water, H₂O.

In an ionic bond, an atom loses or gains electrons, which have a negative charge. An atom that loses one or more electrons becomes positive while an atom that gains electrons becomes negative. Atoms that are positive or negative are known as ions. Positive ions (known as cations) and negative ions (anions) attract and form strong ionic bonds. Sodium and chlorine bond as sodium chloride, or common salt.
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Other types of bonds include metallic bonds and hydrogen bonds. In the first, metal ions' outer electrons float around in a common pool. The second occur between water molecules. The partially positively charged hydrogen atoms are attracted to partially negatively charged oxygen atoms to create weak inter-molecular bonds, known as hydrogen bonds.

Pure substance

A chemical substance cannot be separated by physical means; it can be broken up only by chemical means. It is sometimes called a "pure substance" to distinguish it from a mixture, which can be physically separated into its component parts—often quite easily.

A pure substance is made up of either an element (consisting of the same type of atoms), such as oxygen or sulphur, or a compound (consisting of atoms of more than one element and joined together by chemical bonds), such as water, carbon dioxide or glucose.

Each pure substance has a unique set of chemical and physical properties. Water, for example, is colourless and odourless and has a melting point of 0°C (32°F) and a boiling point of 100°C (212°F). Pure substances exist as solids, liquids or gases and may change between these states as a result of changes in temperature or pressure. A pure substance can only be changed into another by a chemical reaction.

Polymers

One of the main features of the chemical element carbon is that it joins or bonds easily with many other types of atoms and also with itself. Carbon atoms can join together in a chain to form enormously long molecules. Often the chain is made of the same groups of atoms, called sub-units, which are repeated hundreds or thousands of times along its length. This type of molecule is called a polymer and the repeated sub-units are monomers. Many types of plastics and artificial fibres like rayon, acrylic and nylon are polymers. So are many molecules in living things, such as cellulose in plants, chitin in insect body casings and the carrier of genetic information, DNA.