A
absolute configuration
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The spatial arrangement of the atoms about a chiral center.
absolute zero
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The minimum possible temperature: 0 K, -273.15 °C, -459.67 °F.
absorbance
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Absorbance is a measure of the amount of light which is not passed through a substance. It occurs when a material uses the energy from light to change the energy level of the material's electrons. Electrons in atoms or molecules can be "excited" from one energy state to another by the absorption of electromagnetic radiation. The transition of an electron from one state to another is permitted only if the energy of the radiation equals the energy difference between the two states. The wavelength of light indicates how much energy the light has. Since different materials have different electronic energy states, they absorb light of different wavelengths.
absorption spectrum
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A representation of the quantity of light a material absorbs in the form of the absorbance plotted as a function of wavelength, frequency, or energy.
accuracy
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The extent to which an experimental value agrees with true value for a quantity.
acid
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In Arrhenius theory, a species that dissociates to produce hydrogen ions. In Bronsted-Lowry theory, a hydrogen ion donor. In Lewis theory, a species that accepts a pair of electrons to form a covalent bond.
acid dissociation constant
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The equilibrium constant for the dissociation of a weak acid. Also called acid ionization constant.
acid ionization constant
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The equilibrium constant for the ionization of a weak acid. Also called acid dissociation constant.
acid rain
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Rain that is made acidic by the presence of sulfur and nitrogen oxides from the combustion of fossil fuels.
acid-base indicator
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A substance that changes color depending on the concentration of hydrogen ions in solution.
acidic anhydride
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A species (often a non-metal oxide) that reacts with water to form an acid.
actinide
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An element in the periodic table from actinium (atomic number 89) to lawrencium (atomic number 103). Sometimes actinium itself is not included in this series; also called actinoid.
actinoid
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An element in the periodic table from actinium (atomic number 89) to lawrencium (atomic number 103). Sometimes actinium itself is not included in this series; also called actinide.
activated complex
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In the mechanism of a reaction, a species which lies at an energy peak, and that must either fall apart to form products or reform reactants.
activation energy
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The energy barrier over which a reaction must progress in order for reactants to form products.
active metal
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A highly reactive metal, usually an element in the first or second column of the periodic table.
active site
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The location where catalysis occurs in an enzyme or other catalyst.
activity series
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A sequence of species arranged according to their reactivity; often used to describe metals and the ease with which they can be oxidized.
actual yield
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The quantity of a product found to be formed in a chemical reaction (distinguished from theoretical yield).
acyl group
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The functional group RC(=O)-, formally derived from a carboxylic acid by the removal of the hydroxyl group.
addition polymer
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The kind of polymer that is formed by the combination of monomers without the release of a small molecules at the point where monomers are joined.
addition reaction
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A reaction in which two species combine to form one species (distinguished from a substitution reaction).
adduct
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A species formed by the union of two species (usually molecules) held together by a coordinate covalent bond.
adhesive force
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An intermolecular force between unlike molecules, usually occurring where two phases meet.
adsorption
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The attachment of species to a surface (distinguished from absorption).
alcohol
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An organic compound containing the functional group -OH.
aldehyde
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An organic compound containing the functional group -CHO.
alicyclic
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Referring to aliphatic compounds containing rings of carbon atoms.
aliphatic
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Referring to hydrocarbons containing no aromatic rings.
alkali
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Any substance in water solution that is bitter, irritating to the skin, and has a pH value greater than 7.0.
alkali metal
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One of the metals appearing in the first column of the periodic table.
alkaline earth
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One of the metals appearing in the second column of the periodic table.
alkaloid
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A naturally occurring, heterocyclic base containing nitrogen.
alkane
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A hydrocarbon containing no multiple bonds between carbon atoms.
alkene
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A hydrocarbon containing one or more double bonds between carbon atoms. Also called olefin.
alkyl group
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A functional group formally derived from an alkane by the removal of a hydrogen atom.
alkyne
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A hydrocarbon containing one or more triple bonds between carbon atoms.
allotrope
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One of two or more different structural forms for an element.
alloy
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A mixture of two or more metals or of one or more metals with a non-metal; often non-stoichiometric.
alpha particle
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A species consisting of two protons and two neutrons and formed by radioactive decay or nuclear bombardment; identical in composition to the nucleus of a helium-4 atom.
amalgam
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An alloy (solution of metals) containing mercury.
amide
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An organic compound formally derived from a carboxylic acid by the replacement of the hydroxyl group by -NH2, -NHR, or -NR2.
amide linkage
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A carboxylic acid containing an amino group (-NH2). In an alpha amino acid, the amino group is attached to the carbon atom adjacent to the carboxyl group.
amine
-
An organic compound formally derived from ammonia by the replacement of one or more hydrogen atoms by alkyl groups.
amino acid
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A carboxylic acid containing an amino group (-NH2). In an alpha amino acid, the amino group is attached to the carbon atom adjacent to the carboxyl group.
amino group
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The functional group of a nitrogen atom bonded to two hydrogen atoms, -NH2; found in amines and amino acids.
amorphous
-
Having no regular crystalline structure.
amphoteric
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Able to behave as either an acid or a base. Used to describe certain elements near the center of the periodic table which form amphoteric hydroxides.
amu
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Abbreviation for atomic mass unit, a unit for expressing the relative masses of atoms. An atom of carbon-12 has a mass of 12 amu. 1 amu corresponds to 1.662x1024 g.
angstrom
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A unit of length equal to 100 pm or 10-10 m.
anhydride
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A compound that reacts with water to form an acid or a base.
anhydrous
-
Free of water. Often used to describe a solid having no water of crystallization or a solvent from which traces of water have been removed.
anion
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A negatively charged ion, attracted toward the anode in an electrolytic cell.
anode
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The electrode in an electrochemical cell toward which anions are drawn and where oxidation occurs; the positively charged electrode in a vacuum tube.
anodizing
-
The electrolytic deposition of a metal oxide film to serve as a protective layer.
antibonding molecular orbital
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An orbital formed by the overlap of out-of-phase orbitals of different atoms; higher in energy than a bonding molecular orbital.
aqueous
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Describing a solution in which the solvent is water.
aromatic
-
Referring to a hydrocarbon containing benzene-like rings and exhibiting characteristic stability and patterns of reactivity.
Arrhenius acid
-
A substance that dissociates in aqueous solution to produce hydrogen ions.
Arrhenius acid-base theory
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The idea that in aqueous solution acids dissociate to produce hydrogen ions and bases dissociate to produce hydroxide ions.
Arrhenius base
-
A substance that dissociates in aqueous solution to produce hydroxide ions.
Arrhenius equation
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An equation that expresses the logarithmic relationship between the rate constant of a reaction and the reciprocal of the temperature (expressed in K).
atactic
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Having no regularly repeating structural units; said of a polymer.
atmosphere
-
A unit of pressure equal to 101.325 kPa; abbreviated atm. Also, the mixture of gases surrounding the earth.
atom
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The smallest particle of matter that retains the chemical properties of an element.
atomic mass
-
The average mass of the naturally occurring isotopes of an element, taking into account the different natural abundances of the isotopes. Expressed relative to the value of exactly 12 for carbon-12; also called atomic weight.
atomic mass unit
-
A unit for expressing the relative masses of atoms; abbreviated amu. An atom of carbon-12 has a mass of 12 amu.
atomic number
-
The number of protons in the nucleus of an atom; used to define the position of an element in the periodic table; represented by the letter Z.
atomic orbital
-
A mathematical function from which electron density around an atom can be calculated.
atomic spectrum
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The collection of broad and narrow bands of light absorbed by or emitted from atoms of an element.
atomic weight
-
The average mass of the naturally occurring isotopes of an element, taking into account the different natural abundances of the isotopes. Expressed relative to the value of exactly 12 for carbon-12; also called atomic mass.
aufbau
-
A scheme for accounting for the electron configurations of elements in which the electron configuration of one element is determined by adding one electron to the electron configuration of the previous element in the periodic table.
autoionization
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The dissociation of the molecules of solvent into ions.
Avogadro's hypothesis
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The idea that equal volumes of two gases at the same temperature and pressure contain equal numbers of molecules.
Avogadro's number
-
The number of units in a mole, defined by the number of carbon atoms in 12 g of carbon-12 and numerically equal to 6.0220 x 1023
axial
-
Oriented along an axis; in a trigonal bipyramidal structure, lying above or below the trigonal plane.
azeotrope
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A solution of two more substances, usually liquids, that remains unchanged in composition as it boils because the vapor has the same composition as the liquid.
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