transmission in investigating thin
Ülms.
electronegative Describing elements 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 electronegative 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 element. 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 element, has a value 4. Some other values 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 spectroscopy See eels.
electron Ûow The transfer of electrons along a series of carrier molecules in the *electron transport
chain.
electron gas A model of the electrons in a metal or a plasma in which
they are regarded as forming a gas
that interacts with a uniformly distributed background of positive
charge to ensure that the system is
electrically neutral. The electron gas
is analysed theoretically using either
classical or quantum statistical mechanics and the kinetic theory of
gases. The electron-gas model accounts for many properties of metals
and plasmas in a qualitative and approximately quantitative way but
cannot give an accurate quantitative
account of these systems, as this
would require the motions of the
positive ions to be taken into account.
electronic effects Effects by
which the reactivity at one part of a
molecule is affected by electron attraction or repulsion originating in
another part of a molecule. Often
this is called an *inductive effect (or
resonance effect), although sometimes the term ‘inductive effect’ is reserved for an inÛuence transmitted
through chemical bonds and is distinguished from a Üeld effect, which
is transmitted through space. An inductive effect through chemical
bonds was formerly called a mesomeric 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 transitions between the electronic states of
molecules. These transitions correspond to the visible or ultraviolet
regions of the electromagnetic spectrum. There are changes in vibrational and rotational energy when
electronic transitions occur. Consequently there are spectral bands associated with changes in vibrational
motion, with these bands having Üne
structure due to changes in rotational motion. Because electronic
transitions are associated with
changes in vibrational motion the
corresponding spectra are sometimes
called vibrational spectra. The electronic spectra of molecules are used
to obtain information about energy
levels in molecules, interatomic distances, dissociation energies of molecules, and force constants of
chemical bonds.
electron microscope A form of
microscope that uses a beam of electrons instead of a beam of light (as in
the optical microscope) to form a
large image of a very small object. In
electronegative
198
e
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Ülms.
electronegative Describing elements 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 electronegative 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 element. 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 element, has a value 4. Some other values 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 spectroscopy See eels.
electron Ûow The transfer of electrons along a series of carrier molecules in the *electron transport
chain.
electron gas A model of the electrons in a metal or a plasma in which
they are regarded as forming a gas
that interacts with a uniformly distributed background of positive
charge to ensure that the system is
electrically neutral. The electron gas
is analysed theoretically using either
classical or quantum statistical mechanics and the kinetic theory of
gases. The electron-gas model accounts for many properties of metals
and plasmas in a qualitative and approximately quantitative way but
cannot give an accurate quantitative
account of these systems, as this
would require the motions of the
positive ions to be taken into account.
electronic effects Effects by
which the reactivity at one part of a
molecule is affected by electron attraction or repulsion originating in
another part of a molecule. Often
this is called an *inductive effect (or
resonance effect), although sometimes the term ‘inductive effect’ is reserved for an inÛuence transmitted
through chemical bonds and is distinguished from a Üeld effect, which
is transmitted through space. An inductive effect through chemical
bonds was formerly called a mesomeric 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 transitions between the electronic states of
molecules. These transitions correspond to the visible or ultraviolet
regions of the electromagnetic spectrum. There are changes in vibrational and rotational energy when
electronic transitions occur. Consequently there are spectral bands associated with changes in vibrational
motion, with these bands having Üne
structure due to changes in rotational motion. Because electronic
transitions are associated with
changes in vibrational motion the
corresponding spectra are sometimes
called vibrational spectra. The electronic spectra of molecules are used
to obtain information about energy
levels in molecules, interatomic distances, dissociation energies of molecules, and force constants of
chemical bonds.
electron microscope A form of
microscope that uses a beam of electrons instead of a beam of light (as in
the optical microscope) to form a
large image of a very small object. In
electronegative
198
e
www.AzShimi.ir www.AzShimi.com