electron density If P(x,y,z) dx dy dzis the probabil-
ity of finding an electron in the volume element dx dy
dzat the point of a molecular entity with coordinates
x,y,z,then P(x,y,z)is the electron density at this point.
For many purposes (e.g., X-ray scattering, forces on
atoms), the system behaves exactly as if the electrons
were spread out into a continuously distributed charge.
The term has frequently been wrongly applied to nega-
tive CHARGE POPULATION.
See alsoCHARGE DENSITY.
electron detachment The reverse of an ELECTRON
ATTACHMENT.
electron donor (1) A MOLECULAR ENTITYthat can
transfer an electron to another molecular entity or to
the corresponding CHEMICAL SPECIES.
(2) A LEWIS BASE. This use is discouraged.
electron-donor-acceptor complex A term some-
times employed instead of CHARGE-TRANSFER COMPLEX
or LEWIS ADDUCT.
See alsoADDUCT; COORDINATION.
electronegativity Each kind of atom has a certain
attraction for the electrons involved in a chemical
bond. This attraction can be listed numerically on a
scale of electronegativity. Since the element fluorine has
the greatest attraction for electrons in bond-forming, it
has the highest value on the scale. Metals usually have
a low electronegativity, while nonmetals usually have
high electronegativity. When atoms react with one
another, the atom with the higher electronegativity
value will always pull the electrons away from the
atom that has the lower electronegativity value.
electroneutrality principle The principle expresses
the fact that all pure substances carry a net charge of
zero.
electronic effect of substituents: symbols and signs
The INDUCTIVE EFFECThas universally been represented
by the symbol I. This is now commonly taken to
include both through-bond and through-space trans-
mission, but I is also used specifically for through-bond
transmission; through-space transmission is then sym-
bolized as F (for FIELD EFFECT). The symbols for the
influence of substituents exerted through electron delo-
calization have variously been M (MESOMERIC EFFECT),
E (ELECTROMERIC EFFECT), T (TAUTOMERIC EFFECT), C
(conjugative), K (konjugativ), and R (RESONANCE
EFFECT). Since the present fashion is to use the term res-
onance effect,R is the most commonly used symbol,
although M is still seen quite often.
Both the possible sign conventions are in use. The
Ingold sign convention associates ELECTRONEGATIVITY
(relative to the hydrogen atom) with a negative sign,
electropositivity with a positive sign. Thus the nitro
group is described as electron-withdrawing by virtue of
its –I and –M effects; chloro is described as a –I, +M
substituent, etc. For CORRELATION ANALYSISand LIN-
EAR FREE-ENERGY RELATIONships, this convention has
been found inconvenient, for it is in contradiction to
the sign convention for polar substituent constants (σ-
constants). Authors concerned with these fields often
avoid this contradiction by adopting the opposite-sign
convention originally associated with Robinson for
electronic effects. This practice is almost always associ-
ated with the use of R for the electron delocalization
effect: thus the nitro group is a +I, +R substituent;
chloro a +I, –R substituent, etc.electron magnetic resonance spectroscopy(EMR
spectroscopy) See ELECTRON PARAMAGNETIC RESO-
NANCE SPECTROSCOPY.electron microscope(EM) A very large tubular
microscope that focuses a highly energetic electron
beam instead of light through a specimen, resulting in
resolving power thousands of times greater than that of
a regular light microscope. A transmission EM (TEM)
is used to study the internal structure of thin sections of
cells, while a scanning EM (SEM) is used to study the
ultrastructure of surfaces. The transmission electron
microscope was the first type of electron microscope,
developed in 1931 by Max Knoll and Ernst Ruska in
Germany, and was patterned exactly on the light trans-
mission microscope except for the focused beam of92 electron density