Science / Chemistry Glossary
Antipyretic: A substance that can lessen or prevent fever.
Antoine Equation: A simple 3-parameter fit to experimental vapor pressures measured over a restricted temperature range: log P = A - . B, T + C, where A, B, and C are 'Antoine coefficients' that vary from sub . . . View Full Definition
Aprotic Solvent: A solvent that does not act as an acid or as a base; aprotic solvents don't undergo autoprotolysis. Examples are pentane, pet ether, and toluene.
Aqua Regia: A mixture of nitric and hydrochloric acids, usually 1:3 or 1:4 parts HNO3 to HCl, used to dissolve gold.
Aqueous: (aq) A substance dissolved in water.
Arene: A hydrocarbon that contains at least one aromatic ring.
Arginine: (R, C6H14N4O2) Arg An essential amino acid and building block of proteins. Arginine acts as a base under physiological conditions; the double-bonded nitrogen on the end of the side chain rea . . . View Full Definition
Aromatic Compound: A compound containing an aromatic ring. Aromatic compounds have strong, characteristic odors.
Aromatic Ring (Ar): An exceptionally stable planar ring of atoms with resonance structures that consist of alternating double and single bonds, e. g. benzene:.
Arrhenius Equation: In 1889, Svante Arrhenius explained the variation of rate constants with temperature for several elementary reactions using the relationship, k = A exp(-Ea/RT), where the rate constant k is . . . View Full Definition
Aryl (Ar): A molecular fragment or group attached to a molecule by an atom that is on an aromatic ring.
Asparagine: A natural amino acid that is the amide of aspartic acid.
Aspartic Acid: (D,HOOCCH2CH(NH2)COOH) A nonessential amino acid* that is abundant in molasses. The carboxylic acid* group on the side chain is ionized under physiological conditions, making aspartic acid r . . . View Full Definition
Atmosphere (Atm): A unit of pressure, equal to a barometer reading of 760 mm Hg. 1 atmosphere is 101325 pascals and 1.01325 bar.
Atom: An atom is the smallest particle of an element that retains the chemical properties of the element. Atoms are electrically neutral, with a positively charged nucleus that binds one or more e . . . View Full Definition
Atomic Mass Unit: A unit of mass equal to 1/12 the mass of a carbon-12 nucleus, which is 1.660 538 73 × 10-27 kg ± 0.000 000 13 × 10-27 kg [1998 CODATA values]. Abbreviated as amu or u. Sometimes called the d . . . View Full Definition
Atomic Nucleus: A tiny, incredibly dense positively charged mass at the heart of the atom. The nucleus is composed of protons and neutrons (and other particles). It contains almost all of the mass of the at . . . View Full Definition
Atomic Number (Z): The number of protons in an atomic nucleus. The atomic number and the element symbol are two alternate ways to label an element. In nuclide symbols, the atomic number is a leading subscript; . . . View Full Definition
Atomic Orbital: A wavefunction that describes the behavior of an electron in an atom.
Atomic Radius: Metallic radius; covalent radius; atomic radii. Compare with ionic radius. One half the distance between nuclei of atoms of the same element, when the atoms are bound by a single covalent bo . . . View Full Definition
Atomic Theory: An explanation of chemical properties and processes that assumes that tiny particles called atoms are the ultimate building blocks of matter.
Atomic Unit: Compare with Bohr radius and hartree. A system of non-SI units used in quantum chemistry to simplify calculations and mathematical expressions. The definitions of atomic units include physic . . . View Full Definition
Atomic Weight: The average mass of an atom of an element, usually expressed in atomic mass units. The terms mass and weight are used interchangeably in this case. The atomic weight given on the periodic ta . . . View Full Definition
Atto (A): Prefix used in the SI system meaning 'multiply by 10-18'. For example, 3 am means 3× 10-18 meters.
Aufbau Principle: An approximate procedure for writing the ground state electronic configuration of atoms. The configuration of an atom is obtained by inserting one electron into the configuration of the atom . . . View Full Definition
Word of the Day:
Malty: An aromatic sensation created by a moderately volatile set of aldehydes and ketones that produces sensations reminiscent of toasted grains.