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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 ?
This cross reactivity may be lost through heat treatment, thus changing their conformation shape or loosing the thermolabile antigens.
We did not find cross reactivity in samples from the non-endemic area, but in the other 87 serum samples we observed a 9.
With our formulation we obtained the lowest cross reactivity results.
We are extremely pleased to add GLP compliant tissue cross reactivity studies to our growing suite of PhaseZERO[R] services.
This latest patent grant covers several novel hCG specific peptides that were modified to minimize potential cross reactivity to lutropin (hLH), a closely related pituitary hormone.
The test was found to be highly sensitive by accurately detecting all positives and no cross reactivity was observed with specimens from individuals with non-HIV infections such as Hepatitis B, Hepatitis C, syphilis, HTLV-1, HTLV-2 and CMV.
The researchers concluded that the Emit II LSD assay run on the SYVA-30R Analyzer is a convenient and accurate method for high-volume screening of LSD in urine with clinically insignificant cross reactivity to certain compounds associated with drugs-of-abuse.