synthetic resin to make it Ûexible. See
plastics.
plastics Materials that can be
shaped by applying heat or pressure.
Most plastics are made from polymeric synthetic *resins, although a
few are based on natural substances
(e.g. cellulose derivatives or shellac).
They fall into two main classes. Thermoplastic materials can be repeatedly
softened by heating and hardened
again on cooling. Thermosetting materials are initially soft, but change irreversibly to a hard rigid form on
heating. Plastics contain the synthetic resin mixed with such additives as pigments, plasticizers (to
improve Ûexibility), antioxidants and
other stabilizers, and Üllers. See
Chronology overleaf.
plastocyanin A blue coppercontaining protein that is found
in chloroplasts and acts as an electron carrier molecule in the lightdependent reactions of *photosynthesis. Plastocyanin consists of
amino acid groups in association
with a copper molecule which gives
this compound a blue colour.
plastoquinone A quinone, found
in chloroplasts, that functions as
one of the carrier molecules of
the electron transport chain in the
light-dependent reactions of *photosynthesis.
platinum Symbol Pt. A silvery
white metallic *transition element
(see also platinum metals); a.n. 78;
r.a.m. 195.09; r.d. 21.45; m.p. 1772°C;
b.p. 3827±100°C. It occurs in some
nickel and copper ores and is also
found native in some deposits. The
main source is the anode sludge obtained in copper–nickel reÜning. The
element is used in jewellery, laboratory apparatus (e.g. thermocouples,
electrodes, etc.), electrical contacts,
and in certain alloys (e.g. with iridium or rhodium). It is also a hydrogenation catalyst. The element
does not oxidize nor dissolve in
hydrochloric acid. Most of its compounds are platinum(II) or
platinum(IV) complexes.
A
• Information from the WebElements site
platinum black Black Ünely divided platinum metal produced by
vacuum evaporation and used as an
absorbent and a catalyst. See also
adams catalyst.
platinum metals The three members of the second and third transition series immediately proceeding
silver and gold: ruthenium (Ru),
rhodium (Rh), and palladium (Pd);
and osmium (Os), iridium (Ir), and
platinum (Pt). These elements, together with iron, cobalt, and nickel,
were formerly classed as group VIII
of the periodic table. The platinumgroup metals are relatively hard and
resistant to corrosion and are used in
jewellery and in some industrial applications (e.g. electrical contacts).
They have certain chemical similarities that justify classifying them together. All are resistant to chemical
attack. In solution they form a vast
range of complex ions. They also
form coordination compounds with
carbon monoxide and other pibonding ligands. A number of complexes can be made in which a hydrogen atom is linked directly to the
metal. The metals and their organic
compounds have considerable catalytic activity. See also transition elements.
Platonic hydrocarbons Hydrocarbons that have the same molecular
geometry as the Üve Platonic solids,
i.e. the tetrahedron, the cube, the octahedron, the dodecahedron and the
icosahedron. These hydrocarbons
have carbon atoms at the vertices of
421
Platonic hydrocarbons
p
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plastics.
plastics Materials that can be
shaped by applying heat or pressure.
Most plastics are made from polymeric synthetic *resins, although a
few are based on natural substances
(e.g. cellulose derivatives or shellac).
They fall into two main classes. Thermoplastic materials can be repeatedly
softened by heating and hardened
again on cooling. Thermosetting materials are initially soft, but change irreversibly to a hard rigid form on
heating. Plastics contain the synthetic resin mixed with such additives as pigments, plasticizers (to
improve Ûexibility), antioxidants and
other stabilizers, and Üllers. See
Chronology overleaf.
plastocyanin A blue coppercontaining protein that is found
in chloroplasts and acts as an electron carrier molecule in the lightdependent reactions of *photosynthesis. Plastocyanin consists of
amino acid groups in association
with a copper molecule which gives
this compound a blue colour.
plastoquinone A quinone, found
in chloroplasts, that functions as
one of the carrier molecules of
the electron transport chain in the
light-dependent reactions of *photosynthesis.
platinum Symbol Pt. A silvery
white metallic *transition element
(see also platinum metals); a.n. 78;
r.a.m. 195.09; r.d. 21.45; m.p. 1772°C;
b.p. 3827±100°C. It occurs in some
nickel and copper ores and is also
found native in some deposits. The
main source is the anode sludge obtained in copper–nickel reÜning. The
element is used in jewellery, laboratory apparatus (e.g. thermocouples,
electrodes, etc.), electrical contacts,
and in certain alloys (e.g. with iridium or rhodium). It is also a hydrogenation catalyst. The element
does not oxidize nor dissolve in
hydrochloric acid. Most of its compounds are platinum(II) or
platinum(IV) complexes.
A
• Information from the WebElements site
platinum black Black Ünely divided platinum metal produced by
vacuum evaporation and used as an
absorbent and a catalyst. See also
adams catalyst.
platinum metals The three members of the second and third transition series immediately proceeding
silver and gold: ruthenium (Ru),
rhodium (Rh), and palladium (Pd);
and osmium (Os), iridium (Ir), and
platinum (Pt). These elements, together with iron, cobalt, and nickel,
were formerly classed as group VIII
of the periodic table. The platinumgroup metals are relatively hard and
resistant to corrosion and are used in
jewellery and in some industrial applications (e.g. electrical contacts).
They have certain chemical similarities that justify classifying them together. All are resistant to chemical
attack. In solution they form a vast
range of complex ions. They also
form coordination compounds with
carbon monoxide and other pibonding ligands. A number of complexes can be made in which a hydrogen atom is linked directly to the
metal. The metals and their organic
compounds have considerable catalytic activity. See also transition elements.
Platonic hydrocarbons Hydrocarbons that have the same molecular
geometry as the Üve Platonic solids,
i.e. the tetrahedron, the cube, the octahedron, the dodecahedron and the
icosahedron. These hydrocarbons
have carbon atoms at the vertices of
421
Platonic hydrocarbons
p
www.AzShimi.ir www.AzShimi.com