The term “molecule” comes from the Latin word for “mass” or “measure.” In chemistry, a molecule is defined as two or more atoms held together by chemical bonds.
A molecule is the smallest particle of an element or compound that has the chemical properties of that element or compound. Molecules are made up of atoms, which are the basic units of matter. The structure and properties of a molecule are determined by the way its atoms are arranged and bonded together by forces called chemical bonds as mentioned above in the first sentence.
Molecules can be either simple or complex. Simple molecules, such as water (H2O) and carbon dioxide (CO2), consist of only a few atoms bonded together. Complex molecules, such as proteins and DNA, have many atoms bonded together in specific ways to form larger structures with more complex functions. They can also be either organic or inorganic. Organic molecules contain carbon, while inorganic molecules do not. Carbon-based molecules are important in biology because they make up the structure of living things, including DNA and proteins. Inorganic molecules include water and salt.
The study of molecules is called molecular science or chemistry. Molecular science covers a wide range of topics including the structure, behavior, properties, and interactions of molecules with other molecules and with their environment.
Most substances on Earth exist as mixtures of different kinds of molecules rather than pure substances with just one kind of molecule. For example, the air is a mixture made up mostly of nitrogen and oxygen molecules.