Molecule

A molecule is a group of two or more atoms held together by chemical bonds. It is the smallest unit of a substance that still retains the chemical properties of that substance. For example, water is made up of molecules, each of which contains two hydrogen atoms and one oxygen atom, written as H2O.

Molecules vary widely in size and structure. Some are very small, like oxygen gas (O2), while others are extremely large, such as proteins and DNA, which contain thousands of atoms. The way atoms are bonded together and the three-dimensional shape they form have a major effect on how the substance behaves.

Chemical bonding

Atoms in a molecule are held together by covalent bonds, meaning the atoms share electrons. The strength of the bond determines how much heat or energy is needed to break apart the molecule. Other types of bonding, such as ionic bonding, usually produce crystalline structures that cannot be counted as individual molecules in a straightforward way.

Formula and structure

The formula of a molecule shows what types of atoms it contains and how many, for example CO2 or C6H12O6. However, the formula alone is not enough: two substances with the same formula can be different if their atoms are bonded differently. This situation is called isomerism.

Chemical reaction

A chemical reaction occurs when bonds break and new bonds form, thereby creating new molecules. The total number of atoms of each type remains the same before and after a reaction; this is the principle of conservation of matter.