496ELECTRON MICROSCOPE
electron microscope. A microscope in which Use: In recombinant DNA technology.
See transfection.
the source of illumination is a stream of electrons
emanating from a tungsten cathode in a high vacuum
and accelerated by a strong electric impulse (300
electron transport chain. See acceptor con-
kv). The electrons are focused by a series of magnet-
trol.
ic fields that function as lenses in the same way as
glass affects waves of visible light, i.e., the electron
electron-volt. (eV). An extremely small unit
stream curves as it passes through the magnetic
used in measuring the energy of electrons and other
field. Such lenses were developed in Germany by
atomic constituents. It is the energy developed by an
Busch, Knoll, and Tuska in the 1930s, and were
electron in falling through a potential difference of
adapted to microscopy by Zworykin and Hillier at
one V, equivalent to 1.6 × 10
−19
J. The rupture of a
the RCA Laboratories in the early 1940s, when the
carbon-to-carbon bond has been calculated to yield
first commercial instruments were produced. The
approximately 5 eV.
electron microscope is characterized by extremely
high resolving power due to the ultrashort wave-
electrophoresis. Migration of suspended or col-
length of electronic radiation—a small fraction of an
loidal particles in a liquid such as rubber latex due to
angstrom unit (A
˚
). Resolution of 2 A
˚
is possible,
the effect of a potential difference across immersed
which permits determination of the structure of mac-
electrodes. The migration is toward electrodes of
romolecules (DNA) and even observation of large
charge opposite to that of the particles. Most solids,
atoms (uranium). Instruments of the scanning trans-
being negatively charged, migrate to the anode, the
mission type with a resolving power of 0.05 A
˚
were
exception being basic dyes, hydroxide solutions,
developed at the University of Chicago. Two kinds
and colloids that have adsorbed positive ions, all of
of electron microscopies are in general use: the
which are positively charged and migrate to the
transmission type, in which the electrons penetrate
cathode. Migrating particles lose their charge at the
the specimen, and the scanning type introduced in
electrode and generally agglomerate around it. Clay
1970, in which the electrons, condensed to a fine
suspensions can be filtered by means of forced-flow
beam, repeatedly traverse the surface of the speci-
electrophoresis. Electrophoresis is important in the
men, producing a three-dimensional contour effect
study of proteins because the molecules of such
by means of secondary electrons emanating from the
materials act like colloidal particles and their charge
specimen itself. Pictures of astonishing accuracy
is positive or negative according to whether the
have been obtained, especially of surface structures,
surrounding solution is acidic or basic. Thus, the
a matter of great importance in the study of catalysis
acidity of the solution can be used to control the
and other critical phenomena in both industry and
direction in which a protein moves upon electropho-
the biological sciences. A unique combination of
resis. It has been found that electrophoresis can be
these techniques is the scanning transmission elec-
carried out more efficiently under zero-gravity con-
tron microscope (STEM), by means of which col-
ditions in outer space than on earth.
ored motion pictures of uranium atoms on a thin-
Use: Experimentally to separate mixtures of electri-
film carbon substrate have been obtained.
cally charged species such as proteins or nucleic
See optical microscope; resolving power; ultramicro-
acids.
scope; field-ion microscope.
See electrodeposition.
electroplating. The deposition of a thin layer or
electron octet. Group of eight valence elec-
coating of metal (e.g., chromium, nickel, copper,
trons. The most stable configuration of the outer-
silver, etc.) on an object by passing an electric cur-
most, or valency, electron shell of the atom.
rent through an aqueous solution of a salt containing
ions of the element being deposited, for example,
electron paramagnetic resonance. (EPR).
Cu
+2
. The material being plated (usually a metal but
A method of spectroscopic analysis similar to nucle-
often a plastic) constitutes the cathode. The anode is
ar magnetic resonance except that microwave radia-
often composed of the metal being deposited; ideal-
tion is employed instead of radio frequencies. It is
ly it dissolves as the process proceeds. The thin layer
used for studying free radicals, crystalline centers,
deposited is sometimes composed of two or more
transition elements, and structures involving un-
metals, in which case it is an alloy. The solution or
paired electrons.
plating bath contains dissolved salts of all the metals
being deposited. Electrolytic cells are used for this
electrophile. An electron-deficient group with a
process. The anode must be an electrical conductor,
strong tendency to accept electrons from an elec-
but may or may not be of the same chemical compo-
tron-rich group (nucleophile).
sition as the material being deposited, and may or
See nucleophile; Lewis acid.
may not dissolve during the process. The purpose of
electroplating is usually protection of the base metal
electroporation. A process using high-voltage from corrosion. Silver is electroplated on copper for
current to make cell membranes permeable to allow economy reasons; plastics may be electroplated for
the introduction of new DNA. decorative effects.