A chemical structure includes molecular geometry, electronic structure and crystal structure of molecules. Molecular geometry refers to the spatial arrangement of atoms in a molecule and the chemical bonds that hold the atoms together. Molecular geometry can range from the very simple, such as diatomic oxygen or nitrogen molecules, to the very complex, such as protein or DNA molecules. Molecular geometry can be roughly represented using a structural formula. Electronic structure describes the occupation of a molecule's molecular orbitals.
The theory of chemical structure was developed in the 1850s and 1860s by several chemists, including Friedrich August Kekule, Archibald Scott Couper, and Aleksandr Butlerov. These chemists demonstrated that the chemical compounds are not composed of random clusters of atoms and functional groups, but have structures with a definite order formed according the valency (chemistry)|valency of the composing atoms.