High Quality Content by WIKIPEDIA articles! The old quantum theory was a collection of results from the years 1900 1925 which predate modern quantum mechanics. The theory was never complete or self-consistent, but was a collection of heuristic prescriptions which are now understood to be the first quantum corrections to classical mechanics.[1] The Bohr model was the focus of study, and Arnold Sommerfeld[2] made a crucial contribution by quantizing the z-component of the angular momentum, which in the old quantum era was inappropriately called space quantization (Richtungsquantelung).