transmission in investigating thin
Ülms.electronegative Describing el-
ements that tend to gain electrons
and form negative ions. The halogens
are typical electronegative elements.
For example, in hydrogen chloride,
the chlorine atom is more elec-
tronegative than the hydrogen and
the molecule is polar, with negative
charge on the chlorine atom. There
are various ways of assigning values
for the electronegativity of an el-
ement. Mulliken electronegativities
are calculated from E = (I + A)/2,
where I is ionization potential and A
is electron afÜnity. More commonly,
Pauling electronegativities are used.
These are based on bond dissociation
energies using a scale in which
Ûuorine, the most electronegative el-
ement, has a value 4. Some other val-
ues on this scale are B 2, C 2.5, N 3.0,
O 3.5, Si 1.8, P 2.1, S 2.5, Cl 3.0, Br
2.8.
electron energy loss spectros-
copySee eels.
electron ÛowThe transfer of elec-
trons along a series of carrier mol-
ecules in the *electron transport
chain.electron gas A model of the elec-
trons in a metal or a plasma in which
they are regarded as forming a gas
that interacts with a uniformly dis-
tributed background of positive
charge to ensure that the system is
electrically neutral. The electron gas
is analysed theoretically using either
classical or quantum statistical me-
chanics and the kinetic theory of
gases. The electron-gas model ac-
counts for many properties of metals
and plasmas in a qualitative and ap-
proximately quantitative way but
cannot give an accurate quantitative
account of these systems, as this
would require the motions of thepositive ions to be taken into ac-
count.
electronic effects Effects by
which the reactivity at one part of a
molecule is affected by electron at-
traction or repulsion originating in
another part of a molecule. Often
this is called an *inductive effect (or
resonance effect), although some-
times the term ‘inductive effect’ is re-
served for an inÛuence transmitted
through chemical bonds and is dis-
tinguished from a Üeld effect, which
is transmitted through space. An in-
ductive effect through chemical
bonds was formerly called a me-
someric effect (or mesomerism) or an
electromeric effect. It is common to
refer to all effects (through bonds or
through space) as resonance effects.
electronic spectra of molecules
The spectra associated with transi-
tions between the electronic states of
molecules. These transitions corre-
spond to the visible or ultraviolet
regions of the electromagnetic spec-
trum. There are changes in vibra-
tional and rotational energy when
electronic transitions occur. Conse-
quently there are spectral bands asso-
ciated with changes in vibrational
motion, with these bands havingÜne
structure due to changes in rota-
tional motion. Because electronic
transitions are associated with
changes in vibrational motion the
corresponding spectra are sometimes
called vibrational spectra. The elec-
tronic spectra of molecules are used
to obtain information about energy
levels in molecules, interatomic dis-
tances, dissociation energies of mol-
ecules, and force constants of
chemical bonds.electron microscope A form of
microscope that uses a beam of elec-
trons instead of a beam of light (as in
the optical microscope) to form a
large image of a very small object. Inelectronegative 198e