electrons. The antiparticle of the electron is the positron.
electron afÜnity Symbol A. The
energy change occurring when an
atom or molecule gains an electron
to form a negative ion. For an atom
or molecule X, it is the energy released for the electron-attachment reaction
X(g) + e → X
–
(g)
Often this is measured in electronvolts. Alternatively, the molar enthalpy change, ∆H, can be used.
electron capture 1. The formation
of a negative ion by an atom or molecule when it acquires an extra free
electron. 2. A radioactive transformation in which a nucleus acquires
an electron from an inner orbit of
the atom, thereby transforming, initially, into a nucleus with the same
mass number but an atomic number
one less than that of the original nucleus (capture of the electron transforms a proton into a neutron). This
type of capture is accompanied by
emission of an X-ray photon or Auger
electron as the vacancy in the inner
orbit is Ülled by an outer electron.
electron conÜguration See configuration.
electron-deÜcient compound A
compound in which there are fewer
electrons forming the chemical
bonds than required in normal electron-pair bonds. Such compounds use
*multicentre bonds. See borane.
electron diffraction Diffraction of
a beam of electrons by atoms or molecules. The fact that electrons can be
diffracted in a similar way to light
and X-rays shows that particles can
act as waves (see de broglie wavelength). An electron (mass m, charge
e) accelerated through a potential difference V acquires a kinetic energy
mv
2 /2 = eV, where v is the velocity of
the electron (nonrelativistic). Thus,
the momentum (p) of the electron is
√(2eVm). As the de Broglie wavelength (λ) of an electron is given by
h/p, where h is the Planck constant,
then λ = h/√(2eVm). For an accelerating voltage of 3600 V, the wavelength of the electron beam is 0.02
nanometre, some 3 × 10
4 times
shorter than visible radiation.
Electrons then, like X-rays, show
diffraction effects with molecules
and crystals in which the interatomic
spacing is comparable to the wavelength of the beam. They have the
advantage that their wavelength can
be set by adjusting the voltage. Unlike X-rays they have very low penetrating power. The Ürst observation
of electron diffraction was by George
Paget *Thomson in 1927, in an experiment in which he passed a beam
of electrons in a vacuum through a
very thin gold foil onto a photographic plate. Concentric circles
were produced by diffraction of electrons by the lattice. The same year
Clinton J. Davisson (1881–1958) and
Lester Germer (1896–1971) performed a classic experiment in
which they obtained diffraction patterns by glancing an electron beam
off the surface of a nickel crystal.
Both experiments were important
veriÜcations of de Broglie’s theory
and the new quantum theory.
Electron diffraction, because of the
low penetration, cannot easily be
used to investigate crystal structure.
It is, however, employed to measure
bond lengths and angles of molecules
in gases. Moreover, it is extensively
used in the study of solid surfaces
and absorption. The main techniques
are low-energy electron diffraction
(LEED) in which the electron beam is
reÛected onto a Ûuorescent screen,
and high-energy electron diffraction
(HEED) used either with reÛection or
197
electron diffraction
e
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electron afÜnity Symbol A. The
energy change occurring when an
atom or molecule gains an electron
to form a negative ion. For an atom
or molecule X, it is the energy released for the electron-attachment reaction
X(g) + e → X
–
(g)
Often this is measured in electronvolts. Alternatively, the molar enthalpy change, ∆H, can be used.
electron capture 1. The formation
of a negative ion by an atom or molecule when it acquires an extra free
electron. 2. A radioactive transformation in which a nucleus acquires
an electron from an inner orbit of
the atom, thereby transforming, initially, into a nucleus with the same
mass number but an atomic number
one less than that of the original nucleus (capture of the electron transforms a proton into a neutron). This
type of capture is accompanied by
emission of an X-ray photon or Auger
electron as the vacancy in the inner
orbit is Ülled by an outer electron.
electron conÜguration See configuration.
electron-deÜcient compound A
compound in which there are fewer
electrons forming the chemical
bonds than required in normal electron-pair bonds. Such compounds use
*multicentre bonds. See borane.
electron diffraction Diffraction of
a beam of electrons by atoms or molecules. The fact that electrons can be
diffracted in a similar way to light
and X-rays shows that particles can
act as waves (see de broglie wavelength). An electron (mass m, charge
e) accelerated through a potential difference V acquires a kinetic energy
mv
2 /2 = eV, where v is the velocity of
the electron (nonrelativistic). Thus,
the momentum (p) of the electron is
√(2eVm). As the de Broglie wavelength (λ) of an electron is given by
h/p, where h is the Planck constant,
then λ = h/√(2eVm). For an accelerating voltage of 3600 V, the wavelength of the electron beam is 0.02
nanometre, some 3 × 10
4 times
shorter than visible radiation.
Electrons then, like X-rays, show
diffraction effects with molecules
and crystals in which the interatomic
spacing is comparable to the wavelength of the beam. They have the
advantage that their wavelength can
be set by adjusting the voltage. Unlike X-rays they have very low penetrating power. The Ürst observation
of electron diffraction was by George
Paget *Thomson in 1927, in an experiment in which he passed a beam
of electrons in a vacuum through a
very thin gold foil onto a photographic plate. Concentric circles
were produced by diffraction of electrons by the lattice. The same year
Clinton J. Davisson (1881–1958) and
Lester Germer (1896–1971) performed a classic experiment in
which they obtained diffraction patterns by glancing an electron beam
off the surface of a nickel crystal.
Both experiments were important
veriÜcations of de Broglie’s theory
and the new quantum theory.
Electron diffraction, because of the
low penetration, cannot easily be
used to investigate crystal structure.
It is, however, employed to measure
bond lengths and angles of molecules
in gases. Moreover, it is extensively
used in the study of solid surfaces
and absorption. The main techniques
are low-energy electron diffraction
(LEED) in which the electron beam is
reÛected onto a Ûuorescent screen,
and high-energy electron diffraction
(HEED) used either with reÛection or
197
electron diffraction
e
www.AzShimi.ir www.AzShimi.com