In the second half of the nineteenth century, the notion of valence was created, which helped to explain the molecular structure of inorganic and organic compounds. The more recent theories of chemical bonding, such as the Lewis structures (1916), cubical atom (1902), valence bond theory (1927), valence shell electron pair repulsion theory (1958), molecular orbitals (1928), and all modern methods of quantum chemistry, arose from the search for the underlying causes of valence.