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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 ?
The 1770 series, based on a chemically reactive system, offers optimal surface appearance, degassing, and cell-size uniformity.
In the case where molecules are chemically reactive, one simply doesn't have time to study them in bulk samples: They decay away before they can be cooled further to observe interesting states," Zwierlein says.
The system must be either in purely physical mode (pure "IBE" ion beam etching, chemically reactive component without) or in reactive mode (RIBE-mode) can be operated.
They release large amounts of chemically reactive molecules called 'reactive oxygen species.
Focusing primarily on developments resulting from the need for sustainable technologies, contributors in the field discuss such topics as new enzymatic products made from soybean oil containing hydrazinoester, elastin- based protein polymers, chemically reactive peptides and enzyme immobilization and assembly.
The DMS flushes out chemically reactive molecules that cause cellular damage, in much the same way that our bodies use antioxidants to bind to free radicals.
tuberculosis that activates PA-824, making the compound chemically reactive and thus harmful to the bacterium.
Chemically reactive concrete stains and methods of forming these stains have been patented.
Because the shell of the liposome consists of chemically reactive elements, various surface tags can be attached, including antibodies, antigens, oligonucleotide probes, and cell receptors.
Water molecules are chemically reactive species, particularly in dissociated or activated form as may occur in process plasmas.
But the two materials are not chemically reactive, so problems of poor surface wetting, poor dispersion, and poor premix uniformity can arise.
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.

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