A Controversial Molecule: The Early History of Triatomic Hydrogen

The hypothesis of a triatomic hydrogen molecule H₃ originated in the early 1910s in connection with J. J. Thomson's experiments on positive rays. Apparently justified by Bohr's atomic theory, the hypothesis was investigated by many chemists and physicists, some of whom believed to have confirmed the existence of neutral H₃ in the form of ‘active hydrogen’. However, experimental results were contradictory and for this reason the H₃ molecule remained controversial. By the early 1930s theory as well as accumulated experimental evidence spoke against its existence. The paper discusses the fate of the H₃ molecule and the H₃⁺ ion from 1911 to about 1935, when the H₃ hypothesis was effectively abandoned. It also connects this history with the more recent developments that started with the discovery of the H₃ spectrum in 1979 and led to much work on the astronomical significance of the H₃⁺ ion.