blende A naturally occurring metal
sulphide, e.g. zinc blende ZnS.
Bloch’s theorem A theorem relating to the *quantum mechanics of
crystals stating that the wave function ψ for an electron in a periodic
potential has the form ψ(r) =
exp(ik•r)U(r), where k is the wave
vector, r is a position vector, and U(r)
is a periodic function that satisÜes
U(r + R) = U(r), for all vectors R of the
Bravais lattice of the crystal. Block’s
theorem is interpreted to mean that
the wave function for an electron in
a periodic potential is a plane wave
modulated by a periodic function.
This explains why a free-electron
model has some success in describing
the properties of certain metals although it is inadequate to give a
quantitative description of the properties of most metals. Block’s theorem was formulated by the
German-born US physicist Felix
Bloch (1905–83) in 1928. See also energy band.
block See periodic table.
block copolymer See polymer.
blood pigment Any one of a
group of metal-containing coloured
protein compounds whose function
is to increase the oxygen-carrying capacity of blood.
blue vitriol See copper(ii)
sulphate.
boat See ring conformations.
boat conformation See conformation.
BOD See biochemical oxygen demand.
body-centred cubic (b.c.c.) See
cubic crystal.
boehmite A mineral form of a
mixed aluminium oxide and hydroxide, AlO.OH. It is named after the
German scientist J. Böhm. See aluminium hydroxide.
Bohr, Niels Henrik David (1885–
1962) Danish physicist. In 1913 he
published his explanation of how
atoms, with electrons orbiting a
central nucleus, achieve stability by
assuming that their angular momentum is quantized. Movement of electrons from one orbit to another is
accompanied by the absorption or
emission of energy in the form of
light, thus accounting for the series
of lines in the emission *spectrum of
hydrogen. For this work Bohr was
awarded the 1922 Nobel Prize for
physics. See bohr theory.
bohrium Symbol Bh. A radioactive
*transactinide element; a.n. 107. It
was Ürst made in 1981 by Peter Armbruster and a team in Darmstadt,
Germany, by bombarding bismuth209 nuclei with chromium-54 nuclei.
Only a few atoms of bohrium have
ever been detected.
A
• Information from the WebElements site
Bohr magneton See magneton.
Bohr theory The theory published
in 1913 by Niels *Bohr to explain the
line spectrum of hydrogen. He assumed that a single electron of mass
m travelled in a circular orbit of radius r, at a velocity v, around a positively charged nucleus. The angular
momentum of the electron would
then be mvr. Bohr proposed that electrons could only occupy orbits in
which this angular momentum had
certain Üxed values, h/2π, 2h/2π,
3h/2π,…nh/2π, where h is the Planck
constant. This means that the angular momentum is quantized, i.e. can
only have certain values, each of
which is a multiple of n. Each permitted value of n is associated with an
orbit of different radius and Bohr assumed that when the atom emitted
75
Bohr theory
b
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sulphide, e.g. zinc blende ZnS.
Bloch’s theorem A theorem relating to the *quantum mechanics of
crystals stating that the wave function ψ for an electron in a periodic
potential has the form ψ(r) =
exp(ik•r)U(r), where k is the wave
vector, r is a position vector, and U(r)
is a periodic function that satisÜes
U(r + R) = U(r), for all vectors R of the
Bravais lattice of the crystal. Block’s
theorem is interpreted to mean that
the wave function for an electron in
a periodic potential is a plane wave
modulated by a periodic function.
This explains why a free-electron
model has some success in describing
the properties of certain metals although it is inadequate to give a
quantitative description of the properties of most metals. Block’s theorem was formulated by the
German-born US physicist Felix
Bloch (1905–83) in 1928. See also energy band.
block See periodic table.
block copolymer See polymer.
blood pigment Any one of a
group of metal-containing coloured
protein compounds whose function
is to increase the oxygen-carrying capacity of blood.
blue vitriol See copper(ii)
sulphate.
boat See ring conformations.
boat conformation See conformation.
BOD See biochemical oxygen demand.
body-centred cubic (b.c.c.) See
cubic crystal.
boehmite A mineral form of a
mixed aluminium oxide and hydroxide, AlO.OH. It is named after the
German scientist J. Böhm. See aluminium hydroxide.
Bohr, Niels Henrik David (1885–
1962) Danish physicist. In 1913 he
published his explanation of how
atoms, with electrons orbiting a
central nucleus, achieve stability by
assuming that their angular momentum is quantized. Movement of electrons from one orbit to another is
accompanied by the absorption or
emission of energy in the form of
light, thus accounting for the series
of lines in the emission *spectrum of
hydrogen. For this work Bohr was
awarded the 1922 Nobel Prize for
physics. See bohr theory.
bohrium Symbol Bh. A radioactive
*transactinide element; a.n. 107. It
was Ürst made in 1981 by Peter Armbruster and a team in Darmstadt,
Germany, by bombarding bismuth209 nuclei with chromium-54 nuclei.
Only a few atoms of bohrium have
ever been detected.
A
• Information from the WebElements site
Bohr magneton See magneton.
Bohr theory The theory published
in 1913 by Niels *Bohr to explain the
line spectrum of hydrogen. He assumed that a single electron of mass
m travelled in a circular orbit of radius r, at a velocity v, around a positively charged nucleus. The angular
momentum of the electron would
then be mvr. Bohr proposed that electrons could only occupy orbits in
which this angular momentum had
certain Üxed values, h/2π, 2h/2π,
3h/2π,…nh/2π, where h is the Planck
constant. This means that the angular momentum is quantized, i.e. can
only have certain values, each of
which is a multiple of n. Each permitted value of n is associated with an
orbit of different radius and Bohr assumed that when the atom emitted
75
Bohr theory
b
www.AzShimi.ir www.AzShimi.com