Many elements are mixtures of non-radioactive isotopes (see Section 3.1), and in 1934 Harold Urey of Columbia University had been given the Nobel Prize for Chemistry for his isolation of heavy hydrogen (deuterium). Urey had also separated uranium isotopes, and his work was an important basis for the investigations by Otto Hahn from Berlin. In attempts to make transuranium elements, i.e., elements with a higher atomic number than 92 (uranium), by radiating uranium atoms with neutrons, Hahn discovered that one of the products was barium, a lighter element. Lise Meitner, at the time a refugee from Nazism in Sweden, who had earlier worked with Hahn and taken the initiative for the uranium bombardment experiments, provided the explanation, namely, that the uranium atom was cleaved and that barium was one of the products [3]. Hahn was awarded the Nobel Prize for Chemistry in 1944 "for his discovery of the fission of heavy nuclei", and it can be wondered why Meitner was not included. Hahn's original intention with his experiments was later achieved by Edwin M. McMillan and Glenn T. Seaborg of Berkeley, who were given the Nobel Prize for Chemistry in 1951 for "discoveries in the chemistry of transuranium elements".

The use of stable as well as radioactive isotopes have important applications, not only in chemistry, but also in fields as far apart as biology, geology and archeology. In 1943 George de Hevesy from Stockholm received the Nobel Prize for Chemistry for his work on the use of isotopes as tracers, involving studies in inorganic chemistry and geochemistry as well as on the metabolism in living organisms. The prize in 1960 was given to Willard F. Libby of the University of California, Los Angeles (UCLA), for his method to determine the age of various objects (of geological or archeological origin) by measurements of the radioactive isotope carbon-14.

3.7 General Organic Chemistry
Contributions in organic chemistry have led to more Nobel Prizes for Chemistry than work in any other of the traditional branches of chemistry. Like the first prize in this area, that to Emil Fischer in 1902 (see Section 2), most of them have, however, been awarded for advances in the chemistry of natural products and will be treated separately (Section 3.9). Another large group, preparative organic chemistry, has also been given its own section (Section 3.8), and here only the prizes for more general contributions to organic chemistry will be discussed. In 1969 the Nobel Prize for Chemistry went to Sir Derek H. R. Barton from London, and Odd Hassel from Oslo for developing the concept of conformation, i.e. the spatial arrangement of atoms in molecules, which differ only by the orientation of chemical groups by rotation around a single bond. This stereochemical concept rests on the original suggestion by van't Hoff of the tetrahedral arrangement of the four valences of the carbon atom (see Section 2), and most organic molecules exist in two or more stable conformations.

The Nobel Prize for Chemistry in 1975 to Sir John Warcup Cornforth of the University of Sussex and Vladimir Prelog of ETH in Zürich was also based on research in stereochemistry. Not only can a compound have more than one geometric form, but chemical reactions can also have specificity in their stereochemistry, thereby forming a product with a particular three-dimensional arrangement of the atoms. This is especially true of reactions in living organisms, and Cornforth has mainly studied enzyme-catalyzed reactions, so his work borders onto biochemistry (Section 3.12). One of Prelog's main contributions concerns chiral molecules, i.e. molecules that have two forms differing from one another as the right hand does from the left. Stereochemically specific reactions have great practical importance, as many drugs, for example, are active only in one particular geometric form.

Organometallic compounds constitute a group of organic molecules containing one or more carbon-metal bond, and they are thus the organic counterpart to Werner's inorganic coordination compounds (see Section 3.6). In 1952 Ernst Otto Fischer and Sir Geoffrey Wilkinson independently described a completely new group of organometallic molecules, called sandwich compounds. In such compounds a metal ion is bound not to a single carbon atom but is "sandwiched" between two aromatic organic molecules. Fischer and Wilkinson shared the Nobel Prize for Chemistry in 1973.

Work on the interaction of metal ions with organic molecules was also recognized by the prize in 1987, which was shared by Donald J. Cram of UCLA, Jean-Marie Lehn from Strasbourg (and Paris) and Charles J. Pedersen of the Du Pont Company. These three investigators have synthesized molecules with a ring structure, in which the hole in their middle specifically recognizes and binds different metal ions. They can, for example, distinguish between closely related ions, such as those of sodium and potassium, and thus they mimic enzymes in their specificity. The first such compound was synthesized by Pedersen in 1967, and later Lehn and Cram developed increasingly sophisticated organic compounds with cavities and cages in which not only metal ions but other molecules are bound. This research has applications in the whole spectrum of the chemical field, from inorganic chemistry to biochemistry.

George A. Olah from the University of Southern California was awarded the Nobel Prize for Chemistry in 1994 "for his contributions to carbocation chemistry". Already in the 1920s and 1930s chemists had suggested that positively charged ions of hydrocarbons are formed as short-lived intermediates in organic chemical reactions. Such carbocations were, however, thought to be so reactive and unstable that it would be impossible to prepare them in quantity. Olah's investigations, starting in the 1960s, contradicted this supposition, since he showed that stable carbocations can be prepared by the use of a new type of extremely acidic compounds ("superacids"), and carbocation chemistry now has a prominent position in all modern textbooks of organic chemistry.