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Summary Article: Pauling, Linus Carl (1901-1994)
From The Hutchinson Dictionary of Scientific Biography

Place: United States of America

Subject: biography, chemistry

US theoretical chemist and biologist whose achievements ranked among the most important of any in 20th-century science. His main contribution was to molecular structure and chemical bonding. He was one of the very few people to have been awarded two Nobel Prizes: he received the 1954 Nobel Prize for Chemistry (for his work on intermolecular forces) and the 1962 Peace Prize. Throughout his career his work was noted for the application of intuition and inspiration, assisted by his phenomenal memory; he often carried over principles from one field of science and applied them to another.

Pauling was born in Portland, Oregon, on 28 February 1901, the son of a pharmacist. He began his scientific studies at Oregon State Agricultural College, from which he graduated in chemical engineering in 1922. He then began his research at the California Institute of Technology, Pasadena, gaining his PhD in 1925. From 1925 to 1927 he was a postdoctoral fellow in Europe, where he met the chief scientists of the day who were working on atomic and molecular structure: Arnold Sommerfeld in Munich, Niels Bohr in Copenhagen, Erwin Schrödinger in Zürich, and William Bragg (see William and Lawrence Bragg) in London. He became a full professor at Pasadena in 1931 and left there in 1936 to take up the post of director of the Gates and Crellin Laboratories, which he held for the next 22 years. He also held university appointments at the University of California, San Diego, and Stanford University, and during the 1960s spent several years on a study of the problems of war and peace at the Center for the Study of Democratic Institutions at Santa Barbara, California. His last appointment was as director of the Linus Pauling Institute of Science and Medicine at Menlo Park, California.

Pauling's early work reflects his European experiences. In 1931 he published a classic paper, ‘The nature of the chemical bond’, in which he used quantum mechanics to explain that an electron-pair bond is formed by the interaction of two unpaired electrons, one from each of two atoms, and that once paired these electrons cannot take part in the formation of other bonds. It was followed by the book Introduction to Quantum Mechanics (1935), of which he was coauthor. He was a pioneer in the application of quantum mechanical principles to the structures of molecules, relating them to interatomic distances and bond angles by X-ray and electron diffraction, magnetic effects, and thermochemical techniques.

It was Pauling who introduced the concept of hybrid orbitals in molecules to explain the symmetry exhibited by carbon atoms in most of their compounds. The electons in the ground state and in the excited state of the carbon atom can be represented as follows:

One of the 2p electrons can then form sp hybrid orbitals with the 2s electron, with two 2p atomic orbitals remaining.

In acetylene (ethyne), for example, the overlap of two sp hybrid orbitals between two carbon atoms results in a linear molecule. A hydrogen atom is bonded to each end by overlap between the carbons' sp orbitals and the s orbitals of the hydrogens. (The remaining carbon p orbitals also overlap to form two π bonds, which, together with the bond just described, account for the traditional triple bond in this molecule.)

The structures of many other organic molecules can be explained in a similar way.

Pauling also investigated the electronegativity of atoms and polarization (movement of electrons) in chemical bonds. He assigned electronegativities on a scale up to 4.0. A pair of electrons in a bond are pulled preferentially towards an atom with a high electronegativity. In hydrogen chloride, HCl, for example, hydrogen has an electronegativity of 2.1 and chlorine of 3.5. The bonding electrons are pulled towards the chlorine atom, giving it a small excess negative charge (and leaving the hydrogen atom with a small excess positive charge), polarizing the hydrogen-chlorine bond.

Electronegativity values can be used to show why certain substances, such as hydrochloric acid, are acid, whereas others, such as sodium hydroxide, are alkaline.

For compounds whose molecules cannot be represented unambiguously by a single structure, Pauling introduced the idea of resonance hybridization. An example is carbon dioxide, CO2:

The true structure is regarded as an intermediate between two or more theoretically possible structures, which are termed canonical forms.

These and Pauling's other ideas on chemical bonding are fundamental to modern theories of molecular structure. Much of this work was consolidated in his book The Nature of the Chemical Bond (1939).

In the 1940s Pauling turned his attention to the chemistry of living tissues and systems. He applied his knowledge of molecular structure to the complexity of life, principally to proteins in blood. With Robert Corey, he worked on the structures of amino acids and polypeptides. They proposed that many proteins have structures held together with hydrogen bonds, giving them helical shapes. This concept assisted Francis Crick and James Watson in their search for the structure of DNA, which they eventually resolved as a double helix.

In his researches on blood, Pauling investigated immunology and sickle-cell disease. Later work confirmed his hunch that the disease is genetic and that normal haemoglobin and the haemoglobin in abnormal ‘sickle’ cells differ in electrical charge. Throughout the 1940s he studied living materials; he also carried out research on anaesthesia. At the end of this period he published two textbooks, General Chemistry (1948) and College Chemistry (1950), which became academic best-sellers.

Like many of his contemporaries, Pauling became concerned about the proliferation of nuclear weapons and their atmospheric testing during the 1950s. He presented to the United Nations a petition signed by 11,021 scientists from 49 countries urging an end to nuclear-weapons testing, and during the 1960s spent several years on a study of the problems of war and peace at the Center for the Study of Democratic Institutions in Santa Barbara, California. For these efforts he was awarded the Nobel Peace Prize in 1962, the year in which the International Nuclear Test Ban Treaty was signed.

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