‘Electronic structure’ examines the work that went into determining the properties of electrons in atoms. Quantum theory was devised by Max Planck in 1900, and was applied to hydrogen atoms by Niels Bohr in 1913. Bohr hypothesized that electrons existed in set shells around a nucleus and then extrapolated this theory, using chemical rather than physical observations, to other elements. G. N. Lewis hypothesized a cube model of electron arrangement around the nucleus. Despite this not being correct, Lewis concluded that chemical bonding was due to the pairing of electrons, an idea still central to modern chemistry. Finally, Charles Bury determined that electron shells around a nucleus did not need to be filled in a particular order.