is the characteristic range of radiations it emits when it is heated, bombarded by electron or ions, or
absorbs photons. The absorption
spectrum of a substance is produced
by examining, through the substance
and through a spectroscope, a continuous spectrum of radiation. The energies removed from the continuous
spectrum by the absorbing medium
show up as black lines or bands.
With a substance capable of emitting
a spectrum, these are in exactly the
same positions in the spectrum as
some of the lines and bands in the
emission spectrum.
Emission and absorption spectra
may show a continuous spectrum, a
line spectrum, or a band spectrum. A
continuous spectrum contains an unbroken sequence of frequencies over
a relatively wide range; it is produced
by incandescent solids, liquids, and
compressed gases. Line spectra are
discontinuous lines produced by excited atoms and ions as they fall back
to a lower energy level. Band spectra
(closely grouped bands of lines) are
characteristic of molecular gases or
chemical compounds. See also spectroscopy.
speculum An alloy of copper and
tin formerly used in reÛecting telescopes to make the main mirror as it
could be cast, ground, and polished
to make a highly reÛective surface. It
has now been largely replaced by silvered glass for this purpose.
sphalerite (zinc blende) A mineral
form of zinc sulphide, ZnS, crystallizing in the cubic system; the principal
ore of zinc. It is usually yellow-brown
to brownish-black in colour and
occurs, often with galena, in metasomatic deposits and also in hydrothermal veins and replacement
deposits. Sphalerite is mined on
every continent, the chief sources including the USA, Canada, Mexico,
Russia, Australia, Peru, and Poland.
sphalerite structure (zinc-blende
structure) A type of ionic crystal
structure in which the anions have
an expanded face-centred cubic
arrangement with the cations occupying one type of tetrahedral hole.
The coordination number of each
type of ion is 4. Examples of compounds with the sphalerite structure
are ZnS, CuCl, CdS, and InAs.
A
• An interactive version of the structure
spherical top See moment of inertia.
sphingolipid See phospholipid.
spiegel (spiegeleisen) A form of
*pig iron containing 15–30% of manganese and 4–5% of carbon. It is
added to steel in a Bessemer converter as a deoxidizing agent and to
raise the manganese content of steel.
spin (intrinsic angular momentum)
Symbol s. The part of the total angular momentum of a particle, atom,
nucleus, etc., that can continue to
exist even when the particle is apparently at rest, i.e. when its translational motion is zero and therefore
its orbital angular momentum is
zero. A molecule, atom, or nucleus in
a speciÜed energy level, or a particular elementary particle, has a particular spin, just as it has a particular
charge or mass. According to *quantum theory, this is quantized and is
restricted to multiples of h/2π, where
h is the *Planck constant. Spin is
characterized by a quantum number
s. For example, for an electron s =
±½, implying a spin of + h/4π when it
is spinning in one direction and –h/4π
when it is spinning in the other. Because of their spin, particles also
have their own intrinsic magnetic
moments and in a magnetic Üeld the
speculum
498
s
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absorbs photons. The absorption
spectrum of a substance is produced
by examining, through the substance
and through a spectroscope, a continuous spectrum of radiation. The energies removed from the continuous
spectrum by the absorbing medium
show up as black lines or bands.
With a substance capable of emitting
a spectrum, these are in exactly the
same positions in the spectrum as
some of the lines and bands in the
emission spectrum.
Emission and absorption spectra
may show a continuous spectrum, a
line spectrum, or a band spectrum. A
continuous spectrum contains an unbroken sequence of frequencies over
a relatively wide range; it is produced
by incandescent solids, liquids, and
compressed gases. Line spectra are
discontinuous lines produced by excited atoms and ions as they fall back
to a lower energy level. Band spectra
(closely grouped bands of lines) are
characteristic of molecular gases or
chemical compounds. See also spectroscopy.
speculum An alloy of copper and
tin formerly used in reÛecting telescopes to make the main mirror as it
could be cast, ground, and polished
to make a highly reÛective surface. It
has now been largely replaced by silvered glass for this purpose.
sphalerite (zinc blende) A mineral
form of zinc sulphide, ZnS, crystallizing in the cubic system; the principal
ore of zinc. It is usually yellow-brown
to brownish-black in colour and
occurs, often with galena, in metasomatic deposits and also in hydrothermal veins and replacement
deposits. Sphalerite is mined on
every continent, the chief sources including the USA, Canada, Mexico,
Russia, Australia, Peru, and Poland.
sphalerite structure (zinc-blende
structure) A type of ionic crystal
structure in which the anions have
an expanded face-centred cubic
arrangement with the cations occupying one type of tetrahedral hole.
The coordination number of each
type of ion is 4. Examples of compounds with the sphalerite structure
are ZnS, CuCl, CdS, and InAs.
A
• An interactive version of the structure
spherical top See moment of inertia.
sphingolipid See phospholipid.
spiegel (spiegeleisen) A form of
*pig iron containing 15–30% of manganese and 4–5% of carbon. It is
added to steel in a Bessemer converter as a deoxidizing agent and to
raise the manganese content of steel.
spin (intrinsic angular momentum)
Symbol s. The part of the total angular momentum of a particle, atom,
nucleus, etc., that can continue to
exist even when the particle is apparently at rest, i.e. when its translational motion is zero and therefore
its orbital angular momentum is
zero. A molecule, atom, or nucleus in
a speciÜed energy level, or a particular elementary particle, has a particular spin, just as it has a particular
charge or mass. According to *quantum theory, this is quantized and is
restricted to multiples of h/2π, where
h is the *Planck constant. Spin is
characterized by a quantum number
s. For example, for an electron s =
±½, implying a spin of + h/4π when it
is spinning in one direction and –h/4π
when it is spinning in the other. Because of their spin, particles also
have their own intrinsic magnetic
moments and in a magnetic Üeld the
speculum
498
s
www.AzShimi.ir www.AzShimi.com