blende A naturally occurring metal
sulphide, e.g. zinc blende ZnS.
Bloch’s theorem A theorem relat-
ing to the *quantum mechanics of
crystals stating that the wave func-
tion ψfor an electron in a periodic
potential has the form ψ(r) =
exp(ik•r)U(r), where kis the wave
vector, ris a position vector, and U(r)
is a periodic function that satisÜes
U(r+ R) = U(r), for all vectors Rof 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 al-
though it is inadequate to give a
quantitative description of the prop-
erties of most metals. Block’s theo-
rem was formulated by the
German-born US physicist Felix
Bloch (1905–83) in 1928. See also en-
ergy band.
blockSee periodic table.
block copolymerSee polymer.
blood pigment Any one of a
group of metal-containing coloured
protein compounds whose function
is to increase the oxygen-carrying ca-
pacity of blood.
blue vitriolSee copper(ii)
sulphate.
boat See ring conformations.
boat conformation See confor-
mation.
BODSee biochemical oxygen de-
mand.
body-centred cubic (b.c.c.)See
cubic crystal.
boehmiteA mineral form of a
mixed aluminium oxide and hydrox-
ide, AlO.OH. It is named after the
German scientist J. Böhm. See alu-
minium 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 momen-
tum is quantized. Movement of elec-
trons 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.
bohriumSymbol Bh. A radioactive
*transactinide element; a.n. 107. It
wasÜrst made in 1981 by Peter Arm-
bruster and a team in Darmstadt,
Germany, by bombarding bismuth-
209 nuclei with chromium-54 nuclei.
Only a few atoms of bohrium have
ever been detected.
A- Information from the WebElements site
Bohr magnetonSee magneton.
Bohr theoryThe theory published
in 1913 by Niels *Bohr to explain the
line spectrum of hydrogen. He as-
sumed that a single electron of mass
m travelled in a circular orbit of ra-
dius r, at a velocity v, around a posi-
tively charged nucleus. The angular
momentum of the electron would
then be mvr. Bohr proposed that elec-
trons could only occupy orbits in
which this angular momentum had
certainÜxed values, h/2π, 2h/2π,
3 h/2π,...nh/2π, where h is the Planck
constant. This means that the angu-
lar momentum is quantized, i.e. can
only have certain values, each of
which is a multiple of n. Each permit-
ted value of n is associated with an
orbit of different radius and Bohr as-
sumed that when the atom emitted75 Bohr theory
b