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The relative capacity of an atom, molecule, or radical to combine chemically with another atom, molecule, or radical.
A measure of the deviation of a nuclear reactor from the critical state at any instant of time such that positive and negative values correspond to reactors above and below critical, respectively; measured in percent k, millikays, dollars, or in-hours.



a measure of the chemical activity of substances accounting for the variety and rate of the reactions possible for a given substance. For example, noble metals (Au, Pt) and inert gases (He, Ar, Kr, Xe) are chemically inactive; that is, they possess low reactivity. Alkali metals (Li, Na, K, Cs) and halogens (F, Cl, Br, I), on the other hand, are chemically active; that is, they possess high reactivity.

In organic chemistry, alkanes are characterized by low reactivity and therefore can undergo only reactions (radical halo-genation and nitration, dehydrogenation, dissociation with rupture of C—C bond) occurring under such vigorous conditions as elevated temperatures and ultraviolet irradiation. For the alkyl halides, additional reactions, such as dehydrohalogen-ation, nucleophilic substitution for the halogen, and the formation of organomagnesium compounds, are possible under mild conditions. The presence of double or triple bonds or functional groups (hydroxyl —OH, carboxyl —COOH, amino —NH2) in a molecule promotes a further increase in reactivity.

Reactivity is quantitatively expressed by specific rate constants or, in the case of reversible processes, by equilibrium constants. Modern concepts of reactivity are based on the valence-bond theory and on the study of the distribution (and displacement upon action of a reagent) of the electron density in a molecule. Electron displacements are defined qualitatively through inductive and mesomeric effects and quantitatively through quantum-mechanical calculations. The key factor determining the relative reactivity of a number of related compounds is the molecular structure, which encompasses the nature of the substituents, the electronic and steric effect of these substituents on the reaction center, and the geometry of the molecule. Reactivity is dependent on the reaction conditions, for example, the nature of the medium, presence of catalysts or inhibitors, pressure, temperature, and irradiation. Depending on the mechanism of a given reaction, these factors all have a varied and sometimes contrary effect on the reaction rate. The quantitative relationship between rate (or equilibrium) constants within one reaction series may be represented by correlation equations, which describe the change in the constant as a function of the change in a given parameter. Examples are seen in the Hammett-Taft equation, which deals with the effect of a substituent, and the Brønsted equation, which deals with solvent polarity.

References in periodicals archive ?
Almost all fresh fruits, vegetables, and cereal grains are rich in phenolic compounds, which inhibit malignant changes in cells through their antioxidant activity, sweeping up chemically reactive oxygen and other free radicals and thereby protecting the cells from structural damage.
The company develops and manufactures chemically reactive materials that clean up harmful exhaust gases as well as catalysed soot filters that prevent diesel particulates from polluting the air.
He found that silver compounds cause bacteria to produce more reactive oxygen species -- chemically reactive molecules that damage the bacterial cell's DNA and enzymes, as well as the membrane that encloses the cell.
Modern researchers have two explanations for copper's anti-microbial effects - the metal can stimulate production of other chemically reactive molecules that damage bacteria; and it is also directly toxic to the bacteria.
The small spheres also make the material more chemically reactive than it would be in bulk form.
Gutowski says, "The trick is that one functional end of the graft chemical is reactive with appropriate attachment sites of the oxidized polymer surface, while the other functional group on the same graft chemical is chemically reactive with the material to be put in contact with the SICOR-treated surface.
Our laboratory studies of what happens to colloidal iron oxides ingested by protozoans show that protozoan grazers can greatly accelerate the production of dissolved and chemically reactive iron from colloidal iron oxides in seawater.
Polyphenolic compounds in black tea may act as antioxidants in the body by removing chemically reactive oxygen and other free radicals, thereby, helping to protect cells from structural damage.
Having "extra" electrons or electron "vacancies" to fill makes some elements chemically reactive.
Chemical blowing-agent pellet concentrates with loadings of 10-40%, based on exothermic, endothermic or chemically reactive systems, are produced for injection molding, wire and cable, and other extrusion applications.
The production of chemically reactive oxidants on the surface of Mars and icy moons such as Europa may provide clues to their habitability and offers new avenues for future space exploration and the analysis of surface soil and ice shells, according to three Special Papers published in the most recent (Volume 6, Number 3) issue of Astrobiology, a peer-reviewed journal published by Mary Ann Liebert, Inc.

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