noble gases (rare gases) All the elements of the peri-
odic Group 0; also called rare gases; formerly called
inert gases: helium (He), neon (Ne), argon (Ar), kryp-
ton (Kr), xenon (Xe), and radon (Rn).
no-bond resonance SeeHYPERCONJUGATION.
nomenclature SeeBINOMIAL.
nonbonding orbital A MOLECULAR ORBITAL (a
region in space within a molecule where electrons can
be found) occupancy that does not significantly
increase or decrease stability. Often, the main contribu-
tion to the molecular orbital comes only from an
atomic orbital of one atom.
nonclassical carbocation A CARBOCATION the
GROUND STATE of which has delocalized (bridged)
bonding pi or sigma electrons. (Allylic and benzylic
carbocations are not considered nonclassical.)
See alsoDELOCALIZATION.
nonclassical isostere Synonymous with BIOISOSTERE.
noncyclic electron flow The first stage of PHOTO-
SYNTHESIS; begins when light energy enters a cluster of
pigment molecules called the PHOTOSYSTEM, located in
the thylakoid; the light-induced flow of electrons from
water to NADP in oxygen-evolving photosynthesis
involving both photosystems I and II. Photosystems are
a large complex of proteins and chlorophyll that cap-
ture energy from sunlight. Both systems I and II include
special forms of chlorophyll A. Photosystem I, or P-
700, includes chlorophyll A pigment with a specific
absorbance of 700 nm (red light). Photosystem II, or P-
680, contains the reaction center responsible for oxy-
gen evolution and contains a special chlorophyll A that
absorbs light at 680 nm (red light). If the photochemi-
cal reactions in photosystem II are inhibited, photosys-
tem I is inhibited as well.
noncyclic photophosphorylation The formation of
ATPby NONCYCLIC ELECTRON FLOW.
nonelectrolyte Any substance or material that does
not conduct electricity when aqueous.nonpolar covalent bond A covalent bond formed
by the equal sharing of electrons between two atoms
with the same electronegativity. Electronegativity is the
tendency of an atom to attract electrons to itself in a
COVALENT BOND.normal kinetic isotope effect SeeISOTOPE EFFECT.normal mode (of vibration) In molecular vibra-
tions, in a normal mode the atoms all move with the
same frequency and phase; however, the amplitudes
and directions of their motions differ. Generally, any
stable mode or frequency at which the medium can
vibrate independently.n-σdelocalization(n-σno bond resonance) DELO-
CALIZATIONof a free electron pair (n) into an antibond-
ing SIGMA ORBITAL(s).
See alsoHYPERCONJUGATION; RESONANCE.N-terminal amino acid residue See AMINO ACID
RESIDUE.n-to-pi-star transition (n →π*) An electronic tran-
sition in which an electron is excited from a nonbond-
ing orbital to an antibonding pi orbital, occurring in
the UV-visible range.n-type semiconductor A semiconductor where elec-
trical conduction is mostly due to the movement of
electrons.nuclear binding energy Energy produced by the loss
of mass from the formation of an atom from protons,
electrons, and neutrons; energy released in the forma-
tion of an atom from the subatomic particles.194 noble gases