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Linus Carl Pauling (February 28, 1901-August 19, 1994) was an American physical chemist. He won a Nobel Prize in chemistry and a Nobel Peace Prize.

Pauling was born in Portland, Oregon. His father, an unsuccessful druggist, moved his family to a number of different cities in Oregon from 1903 to 1909, finally returning to Portland that year. When the elder Pauling died in 1910 of a perforated ulcer, Linus' mother was left to care for him and two younger siblings.

Pauling was a voracious reader as a child, and at one point his father wrote a letter to a local paper inviting suggestions of additional books that would occupy his time. A friend, Lloyd Jeffress, had a small chemistry laboratory in his bedroom when Pauling was in grammar school, and Jeffress' laboratory experiments inspired Pauling to plan to become a chemical engineer.

In high school, Pauling continued to experiment in chemistry, borrowing much of the equipment and materials from an abandoned steel company near which his grandfather worked as a night watchman.

Pauling failed to take some required American history courses and did not qualify for his high school diploma. The school awarded him the diploma 45 years later, after he had won two Nobel Prizes!

In 1917 Pauling entered the Oregon Agricultural College in Corvallis, now Oregon State University. Because of financial needs, he had to work full-time while attending a full schedule of classes. After his sophomore year, he planned to take a job in Portland to help support his mother, but the college offered him a position teaching quantitative analysis (a course Pauling had just finished taking as a student). This allowed him to continue his studies at OAC.

In his last two years at OAC, Pauling became aware of the work of Gilbert N. Lewis and Irving Langmuir on the electronic structure of atoms and their bonding to form molecules. He decided to focus his research on how the physical and chemical properties of substances are related to the structure of the atoms of which they are composed, becoming one of the founders of the new science of quantum chemistry.

In his senior year he met Ava Helen Miller, a fellow student, and he married her on June 17, 1923; they had three sons and a daughter.

In 1922, Pauling graduated from OAC and went to graduate school at the California Institute of Technology ("CalTech") in Pasadena, California. His graduate research involved the use of X-ray diffraction to determine crystal structure. He published seven papers on the crystal structure of minerals while he was at CalTech. He received his Ph. D. degree, summa cum laude, in 1925.

He then traveled to Europe to study under Arnold Sommerfeld in Munich, Niels Bohr in Copenhagen, and Erwin Schrödinger in Zürich. All three were working in the new field of quantum mechanics. It was while he was studying at OAC that Pauling had first been exposed to quantum mechanics, and he was now interested in seeing how it might help in the understanding of Pauling's chosen field of interest, the electronic structure of atoms and molecules. He devoted the two years of his European trip to this work, and decided to make this the focus of his future research, becoming one of the first scientists in the field of quantum chemistry. In 1927, he hook a new position as an assistant professor at CalTech in theoretical chemistry.

Pauling began his faculty career at CalTech with a very productive five years, both continuing with his X-ray crystal studies and performing quantum mechanical calculations on atoms and molecules. He published approximately fifty papers in those five years. In 1929 he was promoted to associate professor, and in 1930, to full professor. By 1931, the American Chemical Society awarded Pauling the Langmuir Prize for the most significant work in pure science by a person 30 years of age or under.

In the summer of 1930 Pauling made another European trip, learning about the use of electrons in diffraction studies similar to the ones he had performed with X rays. With a student of his, L. O. Brockway, he built an electron diffraction instrument at CalTech and used it to study the molecular structure of a large number of chemical substances.

He introduced the concept of electronegativity in 1932. Using the various properties of molecules, such as the energy required to break bonds and the dipole moments of molecules, he established a scale and an associated numerical value for most of the elements, the Pauling Electronegativity Scale, which is useful in predicting the nature of bonds between atoms in molecules. (Another measure of electronegativity was defined by Robert S. Mulliken; the Mulliken scale generally correlates with Pauling's, but not perfectly. The Pauling scale is the more frequently cited electronegativity scale.)

In the 1930s he began publishing papers on the nature of the chemical bond, leading to his famous textbook on the subject published in 1939. It is based primarily on his work in this rea that he received the Nobel Prize in Chemistry in 1954 "for his research into the nature of the chemical bond and its application to the elucidation of the structure of complex substances".

Part of Pauling's work on the nature of the chemical bond led to his introduction of the concept of hybridization. While it is normal to think of the electrons in an atom as being described by orbitals of types such as s, p, etc., it turns out that in describing the bonding in molecules, it is better to construct functions that partake of some of the properties of each. Thus the one 2s and three 2p orbitals in a carbon atom can be combined to make four equivalent orbitals (called sp hybrid orbitals), which would be the appropriate orbitals to describe carbon compounds such as methane, or the 2s orbital may be combined with two of the 2p orbitals to make three equivalent orbitals (called sp hybrid orbitals), with the remaining 2p orbital unhybridized, which would be the appropriate orbitals to describe certain unsaturated carbon compounds such as ethylene. Other hybridization schemes are also found in other types of molecules.

He received the Nobel Peace Prize in 1962. The presentation speech describes him as "Linus Carl Pauling, who ever since 1946 has campaigned ceaselessly, not only against nuclear weapons tests, not only against the spread of these armaments, not only against their very use, but against all warfare as a means of solving international conflicts."

He is also well-known for his later research into the therapeutic effects of vitamin C. Somewhat controversially, he recommended very high doses of the vitamin as a prophylactic.