P
p block
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Columns 13 through 18 of the periodic table, containing elements in the periodic table over which the p atomic orbitals are being filled.
para
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Describing the relationship between two groups attached to carbon atoms on a benzene-like ring that have two carbon atoms between them.
paramagnetic
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Describing a substance containing unpaired electrons that is attracted by a magnetic field.
partial pressure
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That portion of the total pressure of a mixture of gases contributed by the presence of one of the component gases.
pascal
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The SI unit for pressure, equal to a kg/s2m
Pauli exclusion principle
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The statement that no two electrons in an atom can have the same set of quantum numbers; the principle leads to the rule that only two electrons (having opposite spin) can occupy an atomic orbital.
peptide
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A molecule of two or more amino acids joined by amide linkages or "peptide bonds."
peptide bond
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The amide linkage that joins the carboxylic acid end of one amino acid with the amino end of another amino acid to form a peptide or protein.
percent ionization
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The fraction (expressed as a percent) of an electrolyte that dissociates into ions; this fraction depends upon both the value of the equilibrium constant and the concentration of the species that is dissociating.
period
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Those elements from a single column of the periodic table.
periodic table
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A chart showing the symbols of the elements arranged in order by atomic number and having chemically related elements appearing in columns.
pH
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A logarithmic measure of the concentration of hydrogen ion expressed as -log10([H+]).
phase diagram
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A diagram showing conditions, most typically of pressure and temperature, under which different phases of a substance exist at equilibrium.
photon
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A quantum of energy from electromagnetic radiation; the value of the energy available is the product of the frequency of light and Planck's constant.
phototube detector
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The phototube detector converts light energy to an electrical signal. The intensity of light can be measured with the phototube detector.
physical change
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A process in which the identity of substances remains unchanged.
physical property
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A characteristic of a material that can be measured without changing the material's chemical composition.
pi bond
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A bond in which the electron density lies above and below a plane containing the bonded atoms; formed by the overlap of two p orbitals or a p orbital and a d orbital.
pK
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A logarithmic measure of the value of the equilibrium constant expressed as -log10([K]).
Planck's constant
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A proportionality constant between the energy of a photon of light and the frequency of that light; equal to 6.6261 x 10-34 J sec
pOH
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A logarithmic measure of the concentration of hydroxide ion expressed as -log10([OH-]).
polar
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Describing a molecule having a separation of centers of positive and negative electrical charge that makes the molecule assume certain orientations more than others in an electric field.
polarizability
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The ability of the electrons in a species to change position in response to the presence of an outside electrical field.
pollution
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The contamination of the air, water, and earth by personal, industrial, and farm waste.
polyatomic
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Containing two or more atoms.
polymer
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A large molecule containing a large number of repeating units; a substance formed from such molecules.
polymerization
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The process of forming a polymer from monomers.
polymorphism
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The ability of a substance to exist in more than one crystalline form.
polypeptide
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A polymer of many amino acids joined by amide linkages or "peptide bonds."
polyprotic
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Describing an acid that can donate two or more hydrogen ions ("protons") to a base.
polysaccharide
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A polymer of many units, each of which is a carbohydrate.
positional isomer
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One of two or more compounds that have the same chemical composition but that differ in the location at which functional groups are attached to the backbone of atoms.
positron
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A positively charged particle having the same mass as an electron.
potentiometer
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An instrument for measuring the voltage of a system by opposing that voltage until no current flows.
precipitation
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The formation of a solid within a solution, often by the combination of cations and anions to form an insoluble ionic compound.
precision
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The extent of agreement among several experimental values.
pressure
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Force per unit area; in gases arising from the effect of collisions by the molecules of the gas with the wall of the container.
primary structure
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The sequence of amino acids that defines a protein.
product
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A substance produced by a chemical reaction.
protein
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A biological polymer of amino acids joined by peptide bonds.
proton
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The positively charged particle in an atomic nucleus; its mass is similar to the mass of a hydrogen atom.
pseudo-first-order reaction
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A reaction in which the concentrations of reactants are such that the reaction appears to depend directly upon the concentration of one reactant; the concentrations of other reactants change so little as to affect the rate only negligibly.
pyrophoric
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Igniting spontaneously when exposed to air.
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