Chemistry Dictionary

Terminology "A"


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Absolute Entropy (of a substance)
The increase in the entropy of a substance as it goes from a perfectly ordered crystalline form at 0 °K (where its entropy is zero) to the temperature in question.
Absolute Zero
The zero point on the absolute temperature scale; -273.15°C or 0 K; theoretically, the temperature at which molecular motion ceases.
Absorption Spectrum
Spectrum associated with absorption of electromagnetic radiation by atoms (or other species) resulting from transitions from lower to higher energy states.
Accuracy
How closely a measured value agrees with the correct value.
Acid
A substance that produces H+(aq) ions in aqueous solution. Strong acids ionize completely or almost completely in dilute aqueous solution. Weak acids ionize only slightly.
Acid Anhydride
The oxide of a nonmetal that reacts with water to form an acid.
Acid Anhydride
Compound produced by dehydration of a carbonic acid; general formula is R--C--O--C--R
Acidic Salt
A salt containing an ionizable hydrogen atom; does not necessarily produce acidic solutions.
Activation Energy
Amount of energy that must be absorbed by reactants in their ground states to reach the transition state so that a reaction can occur.
Active Metal
Metal with low ionization energy that loses electrons readily to form cations.
Activity (of a component of ideal mixture)
A dimensionless quantity whose magnitude is: equal to molar concentration in an ideal solution; equal to partial pressure in an ideal gas mixture; and defined as 1 for pure solids or liquids.
Activity Series
A listing of metals (and hydrogen) in order of decreasing activity
Actual Yield
Amount of a specified pure product actually obtained from a given reaction. Compare with Theoretical Yield.
Actinides
Elements 90 to 103 (after actinium)
Acyl Group
Compound derived from a carbonic acid by replacing the --OH group with a halogen (X), usually --Cl; general formula is O R--C--X
Addition Reaction
A reaction in which two atoms or groups of atoms are added to a molecule, one on each side of a double or triple bond
Adhesive Forces
Forces of attraction between a liquid and another surface.
Adsorption
Adhesion of a species onto the surfaces of particles
Alcohol
Hydrocarbon derivative containing an --OH group attached to a carbon atom not in an aromatic ring.
Aldehyde
Compound in which an alkyl or aryl group and a hydrogen atom are attached to a carbonyl group and a hydrogen atom are attached to a carbonyl group; general formula, O-R-C-H
Alkali Metals
Metals of Group IA (Na, K, Rb).
Alkaline Battery
A dry cell in which the electrolyte contains KOH.
Alkaline Earth Metals
Group IIA metals
Alkenes (Olefins)
Unsaturated hydrocarbons that contain one or more carbon-carbon double bonds.
Alkyl Group
A group of atoms derived from an alkane by the removal of one hydrogen atom.
Alkylbenzene
A compound containing an alkyl group bonded to a benzene ring.
Alkynes
Unsaturated hydrocarbons that contain one or more carbon-carbon triple bonds.
Allotropes
Different forms of the same element in the same physical state.
Allotropic Modifications (Allotropes)
Different forms of the same element in the same physical state.
Alloying
Mixing of metal with other substances (usually other metals) to modify its properties.
Alpha Particle
A helium nucleus.
Alpha (a) Particle
Helium ion with 2+ charge; an assembly of two protons and two neutrons.
Alums
Hydrated sulfates of the general formula M+M3+(SO4)2.12H2).
Amide
Compound containing the O-C-N group.
Compound that can be considered a derivative of ammonia in which one or more hydrogens are replaced by a alkyl or aryl groups.
Amine
Derivatives of ammonia in which one or more hydrogen atoms have been replaced by organic groups.
Amine Complexes
Complex species that contain ammonia molecules bonded to metal ions.
Amino Acid
Compound containing both an amino and a carboxylic acid group.The --NH2 group.
For more information see:Amino Acids
Amorphous Solid
A noncrystalline solid with no well-defined ordered structure.
Ampere
Unit of electrical current; one ampere equals one coulomb per second.
Amphiprotism
Ability of a substance to exhibit amphiprotism by accepting donated protons.
Amphoterism
The ability to react with both acids and bases.
Ability of substance to act as either an acid or a base.
Anion
A negative ion; an atom or goup of atoms that has gained one or more electrons.
Anode
In a cathode ray tube, the positive electrode.
Electrode at which oxidation occurs.
Antibonding Orbital
A molecular orbital higher in energy than any of the atomic orbitals from which it is derived; lends instability to a molecule or ion when populated with electrons; denoted with a star (*) superscript or symbol.
Aromatic Hydrocarbons
Benzene and its derivatives.
Artificial Transmutation
An artificially induced nuclear reaction caused by the bombardment of a nucleus with subatomic particiles or small nucei.
Aryl Group
Group of atoms remaining after a hydrogen atom is removed from the aromatic system.
Associated Ions
Short-lived species formed by the collision of dissolved ions of opposite charges.
Atmosphere
A unit of pressure; the pressure that will support a column of mercury 760 mm high at 0 °C.
Atom
The smallest particle of an element
Atomic Mass Unit (amu)
One twelfth of a mass of an atom of the carbon-12 isotope; a unit used for stating atomic and formula weights; also called dalton.
Atomic Number
Integral number of protons in the nucleus; defines the identity of element.
Atomic Orbital
Region or volume in space in which the probability of finding electrons is highest.
Atomic Radius
Radius of an atom.
Atomic Weight
Weighted average of the masses of the constituent isotopes of an element; The relative masses of atoms of different elements.
Aufbau ('building up') Principle
Describes the order in which electrons fill orbitals in atoms.
Autoionization
An ionization reaction between identical molecules.
Avogadro's Law
At the same temperature and pressure, equal volumes of all gases contain the same number of molecules.
Avogadro's Number
The number (6.022x10^23) of atoms, molecules or particles found in exactly 1 mole of substance.