ABSTRACT
Ion chemistry is a product of the 20th century. J J Thomson discovered the electron in 1897 and identified it as a constituent of all matter. Free positive ions (as distinct from ions deduced to exist in solids or electrolytes) were first produced by Thomson just before the turn of the century. He produced beams of light ions, and measured their mass-to-charge ratios, in the early 1900s, culminating in the discovery of two isotopes of neon in 1912 [1]. This year also marked Thomson’s discovery of H+3, which turns out to be the single most important astrophysical ion and which may be said to mark the beginning of the study of the chemistry of ions. Thomson noted that ‘the existence of this substance is interesting from a chemical point of view’, and the problem of its structure soon attracted the distinguished theorist Niels Bohr [2]. (In 1925, the specific reaction producing H+3 was recognized [2].) The mobilities of electrons and ions drifting in weak electric fields were first measured by Thomson, Rutherford and Townsend at the Cavendish Laboratory of Cambridge University in the closing years of the 19th century. The average mobility of the negative charge carrier was observed to increase dramatically in some gases, while the positive charge carrier mobility was unchangedthe anomalous mobility problem-which led to the hypothesis of electron attachment to molecules to form negative ions [3]. In 1936, Eyring, Hirschfelder and Taylor calculated the rate constant for an ion-molecule reaction (the production of H+3!), showing it to be 100 times greater than for a typical neutral reaction, but it was not until 20 years later that any ion-molecule rate constant was measured experimentally [4]. Negative ion-molecule reactions were not studied at all until 1957 [5].
