<P> In 1913, Henry Moseley, working from Van den Broek's earlier idea, introduces concept of atomic number to fix inadequacies of Mendeleev's periodic table, which had been based on atomic weight . The peak of Frederick Soddy's career in radiochemistry was in 1913 with his formulation of the concept of isotopes, which stated that certain elements exist in two or more forms which have different atomic weights but which are indistinguishable chemically . He is remembered for proving the existence of isotopes of certain radioactive elements, and is also credited, along with others, with the discovery of the element protactinium in 1917 . In 1913, J.J. Thomson expanded on the work of Wien by showing that charged subatomic particles can be separated by their mass - to - charge ratio, a technique known as mass spectrometry . </P> <P> American physical chemist Gilbert N. Lewis laid the foundation of valence bond theory; he was instrumental in developing a bonding theory based on the number of electrons in the outermost "valence" shell of the atom . In 1902, while Lewis was trying to explain valence to his students, he depicted atoms as constructed of a concentric series of cubes with electrons at each corner . This "cubic atom" explained the eight groups in the periodic table and represented his idea that chemical bonds are formed by electron transference to give each atom a complete set of eight outer electrons (an "octet"). </P> <P> Lewis's theory of chemical bonding continued to evolve and, in 1916, he published his seminal article "The Atom of the Molecule", which suggested that a chemical bond is a pair of electrons shared by two atoms . Lewis's model equated the classical chemical bond with the sharing of a pair of electrons between the two bonded atoms . Lewis introduced the "electron dot diagrams" in this paper to symbolize the electronic structures of atoms and molecules . Now known as Lewis structures, they are discussed in virtually every introductory chemistry book . </P> <P> Shortly after publication of his 1916 paper, Lewis became involved with military research . He did not return to the subject of chemical bonding until 1923, when he masterfully summarized his model in a short monograph entitled Valence and the Structure of Atoms and Molecules . His renewal of interest in this subject was largely stimulated by the activities of the American chemist and General Electric researcher Irving Langmuir, who between 1919 and 1921 popularized and elaborated Lewis's model . Langmuir subsequently introduced the term covalent bond . In 1921, Otto Stern and Walther Gerlach establish concept of quantum mechanical spin in subatomic particles . </P>

What is the main reason chemistry developed over time