Molecules may be formed by two or more atoms of the same element, as in the oxygen molecule, or they can contain atoms of several different elements. Substances made up of molecules containing more than one kind of element are called chemical compounds. A molecule of water has two hydrogen atoms and one oxygen atom. Water, therefore, is a compound while oxygen is a pure element. Scientists use formulas to represent molecules. Molecular formulas indicate the number and type of atoms that make up a molecule. They use the symbol for the chemical element of each atom in the molecule along with a number to indicate how many of each atom the molecule contains. For instance, a molecule of water is written as H₂O.

Molecules are formed through the interaction of particles called electrons that are found in atoms. Electrons are present either alone or in pairs in outer shells of each atom. When two atoms with unpaired electrons approach each other, the electrons join together to form a pair that is then shared by both atoms. The shared electrons form a bond called a covalent bond that holds the molecule together. These bonds are very strong and require a good deal of energy to break.

Molecules can take on a variety of shapes, including rings or chains. Molecules that fit together in very long chains are called polymers. Sometimes two or more different compounds have the same molecular formula—the same numbers of the same kinds of atoms. The molecules differ from each other in the way the atoms are arranged. Sets of molecules like these are called isomers.

The molecular weight of a molecule is determined by adding together the atomic weights of the atoms that make up the molecule. For example, a water molecule is made of two hydrogen atoms and one oxygen atom. The atomic weight of hydrogen is 1. The atomic weight of oxygen is approximately 16. Thus, the molecular weight of water is (1 × 2) + 16 = 18.