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2389) Robert S. Mulliken

Gist:

Work

The world around us consists of molecules that are composed of atoms. In Niels Bohr’s atomic model, which is based on principles of quantum physics, electrons circle the atomic nucleus in different shells that contain a fixed number of electrons. The assumption was that attractive forces between the atoms in a molecule are the result of atoms sharing electrons to fill the electron shells. Beginning in the mid-1920s, Robert Mulliken applied quantum mechanics to the development of sophisticated models for the movement of electrons within a molecule, so-called molecular orbitals.

Summary

Robert Sanderson Mulliken (born June 7, 1896, Newburyport, Mass., U.S.—died Oct. 31, 1986, Arlington, Va.) was an American chemist and physicist who received the 1966 Nobel Prize for Chemistry for “fundamental work concerning chemical bonds and the electronic structure of molecules.”

A graduate of the Massachusetts Institute of Technology, Mulliken worked, during World War I and for a few years afterward, in government chemical research. He then studied under the physicist Robert A. Millikan at the University of Chicago, receiving his Ph.D. in 1921. He taught at New York University (1926–28) and then joined the faculty of the University of Chicago (1928–85).

Mulliken began working on his theory of molecular structure in the 1920s. He theoretically systematized the electron states of molecules in terms of molecular orbitals. Departing from the idea that electron orbitals for atoms are static and that atoms combine like building blocks to form molecules, he proposed that, when molecules are formed, the atoms’ original electron configurations are changed into an overall molecular configuration. Further extending his theory, he developed (1952) a quantum-mechanical theory of the behaviour of electron orbitals as different atoms merge to form molecules.

During World War II Mulliken worked on the Plutonium Project, part of the development of the atomic bomb, at the University of Chicago. In 1955 he served as scientific attaché at the U.S. embassy in London.

Details

Robert Sanderson Mulliken ForMemRS[1] (June 7, 1896 – October 31, 1986) was an American physical chemist, primarily responsible for the early development of molecular orbital theory, i.e. the elaboration of the molecular orbital method of computing the structure of molecules. Mulliken received the Nobel Prize in Chemistry in 1966 and the Priestley Medal in 1983.[2]

Early years

Robert Mulliken was born in Newburyport, Massachusetts on June 7 1896. His father, Samuel Parsons Mulliken, was a professor of organic chemistry at the Massachusetts Institute of Technology. As a child, Robert Mulliken learned the name and botanical classification of plants and, in general, had an excellent, but selective, memory. For example, he learned German well enough to skip the course in scientific German in college, but could not remember the name of his high school German teacher. He also made the acquaintance, while still a child, of the physical chemist Arthur Amos Noyes.

Mulliken helped with some of the editorial work when his father wrote his four-volume text on organic compound identification, and thus became an expert on organic chemical nomenclature.

Education

In high school in Newburyport, Mulliken followed a scientific curriculum. He graduated in 1913 and succeeded in getting a scholarship to MIT which had earlier been won by his father. Like his father, he majored in chemistry. Already as an undergraduate, he conducted his first publishable research: on the synthesis of organic chlorides. Because he was unsure of his future direction, he included some chemical engineering courses in his curriculum and spent a summer touring chemical plants in Massachusetts and Maine. He received his B. S. degree in chemistry from MIT in 1917.

Early career

At this time, the United States had just entered World War I, and Mulliken took a position at American University in Washington, D.C., making poison gas under James B. Conant. After nine months, he was drafted into the Army's Chemical Warfare Service, but continued on the same task. His laboratory techniques left much to be desired, and he was out of service for months with burns. Later, he contracted a bad case of influenza, and was still hospitalized at war's end.

After the war, he took a job investigating the effects of zinc oxide and carbon black on rubber, but quickly decided that this was not the kind of chemistry he wanted to pursue. Hence, in 1919 he entered the Ph.D. program at the University of Chicago.

Graduate and early postdoctoral education

Mulliken got his doctorate in 1921 based on research into the separation of isotopes of mercury by evaporation, and continued in his isotope separation by this method. While at Chicago, he took a course under the Nobel Prize-winning physicist Robert A. Millikan, which exposed him to the old quantum theory. He also became interested in strange molecules after exposure to work by Hermann I. Schlesinger on diborane.

At Chicago, he had received a grant from the National Research Council (NRC) which had paid for much of his work on isotope separation. The NRC grant was extended in 1923 for two years so he could study isotope effects on band spectra of such diatomic molecules as boron nitride (BN) (comparing molecules with B10 and B11). He went to Harvard University to learn spectrographic technique from Frederick A. Saunders and quantum theory from E. C. Kemble. At the time, he was able to associate with J. Robert Oppenheimer and many future Nobel laureates, including John H. Van Vleck and Harold C. Urey. He also met John C. Slater, who had worked with Niels Bohr.

In 1925 and 1927, Mulliken traveled to Europe, working with outstanding spectroscopists and quantum theorists such as Erwin Schrödinger, Paul A. M. Dirac, Werner Heisenberg, Louis de Broglie, Max Born, and Walther Bothe (all of whom eventually received Nobel Prizes) and Friedrich Hund, who was at the time Born's assistant. They all, as well as Wolfgang Pauli, were developing the new quantum mechanics that would eventually supersede the old quantum theory. Mulliken was particularly influenced by Hund, who had been working on quantum interpretation of band spectra of diatomic molecules, the same spectra which Mulliken had investigated at Harvard. In 1927 Mulliken worked with Hund and as a result developed his molecular orbital theory, in which electrons are assigned to states that extend over an entire molecule. In consequence, molecular orbital theory was also referred to as the Hund-Mulliken theory.

Early scientific career

From 1926 to 1928, he taught in the physics department at New York University (NYU). This was his first recognition as a physicist. Though his work had been considered important by chemists, it clearly was on the borderline between the two sciences and both would claim him from this point on. Then he returned to the University of Chicago as an associate professor of physics, being promoted to full professor in 1931. He ultimately held a position jointly in both the physics and chemistry departments. At both NYU and Chicago, he continued to refine his molecular-orbital theory.

Up to this point, the primary way to calculate the electronic structure of molecules was based on a calculation by Walter Heitler and Fritz London on the hydrogen molecule (H2) in 1927. With the conception of hybridized atomic orbitals by John C. Slater and Linus Pauling, which rationalized observed molecular geometries, the method was based on the premise that the bonds in any molecule could be described in a manner similar to the bond in H2, namely, as overlapping atomic orbitals centered on the atoms involved. Since it corresponded to chemists' ideas of localized bonds between pairs of atoms, this method (called the Valence-Bond (VB) or Heitler-London-Slater-Pauling (HLSP) method), was very popular. In attempting to calculate the properties of excited states (molecules that have been excited by an energy source), the VB method does not always work well. With its description of the electron wave functions in molecules as delocalized molecular orbitals that possess the same symmetry as the molecule, Hund and Mulliken's molecular-orbital method, including contributions by John Lennard-Jones, proved to be more flexible and applicable to a vast variety of types of molecules and molecular fragments, and has eclipsed the valence-bond method. As a result of this development, he received the Nobel Prize in Chemistry in 1966.

Mulliken became a member of the National Academy of Sciences in 1936, the youngest member in the organization's history at the time. He was elected to the American Philosophical Society in 1940 and the American Academy of Arts and Sciences in 1965. He was elected a Foreign Member of the Royal Society (ForMemRs) in 1967.

Mulliken population analysis is named after him, a method of assigning charges to atoms in a molecule.

Personal life

On December 24, 1929, he married Mary Helen von Noé, daughter of Adolf Carl Noé, a geology professor at the University of Chicago. They had two daughters.

Later years

In 1934, he derived a new scale for measuring the electronegativity of elements, which he defined as the average of an atom's ionization enthalpy and electron affinity. This does not entirely correlate with the scale of Linus Pauling, but is generally in close correspondence.

In World War II, from 1942 to 1945, he directed the Information Office for the University of Chicago's Plutonium project. Afterward, he developed mathematical formulas to enable the progress of the molecular-orbital theory.

In 1952. he began to apply quantum mechanics to the analysis of the reaction between Lewis acid and base molecules. In 1961, he became Distinguished Professor of Physics and Chemistry at Florida State University, and continued in his studies of molecular structure and spectra, ranging from diatomic molecules to large complex aggregates. In 1981, Mulliken became a founding member of the World Cultural Council. In 1983, Mulliken received the Golden Plate Award of the American Academy of Achievement. He retired in 1985. His wife died in 1975.

At the age of 90, Mulliken died of congestive heart failure at his daughter's home in Arlington County, Virginia on October 31, 1986. His body was returned to Chicago for burial.

2441) Heavy Water

Gist

Heavy water, or deuterium oxide (D2O), is a form of water where the hydrogen atoms are replaced with deuterium, a stable isotope of hydrogen with an extra neutron. It is denser than ordinary water and has a higher boiling point and freezing point. Heavy water's primary use is as a moderator and coolant in nuclear reactors, but it also has other applications in fields like medicine and life sciences.

Heavy water is a form of water with a unique atomic structure and properties coveted for the production of nuclear power and weapons. Like ordinary water—H20—each molecule of heavy water contains two hydrogen atoms and one oxygen atom. The difference, though, lies in the hydrogen atoms.

Summary

Heavy water (deuterium oxide, 2H2O, D2O) is a form of water in which hydrogen atoms are all deuterium (2H or D, also known as heavy hydrogen) rather than the common hydrogen-1 isotope (1H, also called protium) that makes up most of the hydrogen in normal water. The presence of the heavier isotope gives the water different nuclear properties, and the increase in mass gives it slightly different physical and chemical properties when compared to normal water.

Deuterium is a heavy hydrogen isotope. Heavy water contains deuterium atoms and is used in nuclear reactors. Semiheavy water (HDO) is more common than pure heavy water, while heavy-oxygen water is denser but lacks unique properties. Tritiated water is radioactive due to tritium content.

Heavy water has different physical properties from regular water, such as being 10.6% denser and having a higher melting point. Heavy water is less dissociated at a given temperature, and it does not have the slightly blue color of regular water. It can taste slightly sweeter than regular water, though not to a significant degree. Heavy water affects biological systems by altering enzymes, hydrogen bonds, and cell division in eukaryotes. It can be lethal to multicellular organisms at concentrations over 50%. However, some prokaryotes like bacteria can survive in a heavy hydrogen environment. Heavy water can be toxic to humans, but a large amount would be needed for poisoning to occur.

The most cost-effective process for producing heavy water is the Girdler sulfide process. Heavy water is used in various industries and is sold in different grades of purity. Some of its applications include nuclear magnetic resonance, infrared spectroscopy, neutron moderation, neutrino detection, metabolic rate testing, neutron capture therapy, and the production of radioactive materials such as plutonium and tritium.

Details

Heavy water, or deuterium oxide (D2O), is a form of water where the hydrogen atoms are replaced with deuterium, a stable isotope of hydrogen with an extra neutron. It is denser than ordinary water and has a higher boiling point and freezing point. Heavy water's primary use is as a moderator and coolant in nuclear reactors, but it also has other applications in fields like medicine and life sciences.

Heavy water (D2O) is water composed of two atoms of deuterium, the hydrogen isotope with a mass double that of ordinary hydrogen, and one atom of oxygen. (Ordinary water has a composition represented by H2O.) Thus, heavy water has a molecular weight of about 20 (the sum of twice the atomic weight of deuterium, which is 2, plus the atomic weight of oxygen, which is 16), whereas ordinary water has a molecular weight of about 18 (twice the atomic weight of ordinary hydrogen, which is 1, plus oxygen, which is 16).

As obtained from most natural sources, ordinary water contains about one deuterium atom for every 6,760 ordinary hydrogen atoms. Continued electrolysis of hundreds of liters of water until only a few milliliters remain yields practically pure deuterium oxide. This operation, until 1943 the only large-scale method used, has been superseded by less expensive processes, such as the Girdler sulfide process (deuterium is exchanged between hydrogen sulfide [H2S] and water) and fractional distillation (D2O becomes concentrated in the liquid residue because it is less volatile than H2O). The heavy water produced is used as a moderator of neutrons in nuclear power plants. In the laboratory heavy water is employed as an isotopic tracer in studies of chemical and biochemical processes.

Additional Information

Deuterium oxide, also known as “heavy water” or “deuterium water”, is the compound of oxygen and the heavy isotope of hydrogen, called deuterium. Physically and chemically, heavy water is almost identical to ordinary “light” water, H2O. It is called heavy water because its density is greater than H2O. Its chemical formula is D2O.

Deuterium contains one neutron and one proton in its nucleus, which makes it twice as heavy as protium (hydrogen), which contains only one proton. Deuterium oxide is a colorless and odorless liquid at normal temperature and pressure. Compared to ordinary water, its chemical characteristic is relatively inactive with a specific gravity of 1.10775 (at 25℃), melting/freezing point of 3.82℃, and a boiling point of 101.42℃. The hydrogen bond strength and degree of association between heavy water molecules are both stronger than that of ordinary water molecules.

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