‘Electronic structure’ examines the work that went into determining the properties of electrons in atoms. Quantum theory was devised by Max Planck, and was applied to hydrogen atoms by Niels Bohr. He hypothesised that electrons existed in set shells around a nucleus. He then extrapolated this theory, using chemical rather than physical observations, to other elements. G.N. Lewis hypothesised a cube model of electron arrangement which explained the groups of eight seen in the periodic table. When bonding, the electrons were paired, an idea still central to modern chemistry. Finally, Charles Bury determined that the shells around a nucleus did not need to be filled in order.