However, silicon is used by some organisms in building outer skeletons or shells. The carbon-containing molecules that make up living things are called organic compounds . Organic compounds are often very large and are usually held together by covalent bonds. The study of organic molecules and their interactions is called organic chemistry. The study of molecules and substances not containing carbon is called inorganic chemistry.
Inorganic compounds are often quite small and are usually held together by ionic bonds.
These elements are listed in Table 1 and are compared according to their frequency in the Earth's crust.