synthetic resin to make itÛexible. See
plastics.
plasticsMaterials that can be
shaped by applying heat or pressure.
Most plastics are made from poly-
meric synthetic *resins, although a
few are based on natural substances
(e.g. cellulose derivatives or shellac).
They fall into two main classes. Ther-
moplastic materials can be repeatedly
softened by heating and hardened
again on cooling. Thermosetting ma-
terialsare initially soft, but change ir-
reversibly to a hard rigid form on
heating. Plastics contain the syn-
thetic resin mixed with such addi-
tives as pigments, plasticizers (to
improveÛexibility), antioxidants and
other stabilizers, andÜllers. See
Chronology overleaf.
plastocyanin A blue copper-
containing protein that is found
in chloroplasts and acts as an elec-
tron carrier molecule in the light-
dependent reactions of *photo-
synthesis. Plastocyanin consists of
amino acid groups in association
with a copper molecule which gives
this compound a blue colour.
plastoquinoneA quinone, found
in chloroplasts, that functions as
one of the carrier molecules of
the electron transport chain in the
light-dependent reactions of *photo-
synthesis.
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 ob-
tained in copper–nickel reÜning. The
element is used in jewellery, labora-
tory apparatus (e.g. thermocouples,
electrodes, etc.), electrical contacts,
and in certain alloys (e.g. with irid-
ium or rhodium). It is also a hy-
drogenation catalyst. The element
does not oxidize nor dissolve in
hydrochloric acid. Most of its com-
pounds are platinum(II) or
platinum(IV) complexes.
A- Information from the WebElements site
platinum blackBlackÜnely di-
vided platinum metal produced by
vacuum evaporation and used as an
absorbent and a catalyst. See also
adams catalyst.platinum metalsThe three mem-
bers of the second and third transi-
tion series immediately proceeding
silver and gold: ruthenium (Ru),
rhodium (Rh), and palladium (Pd);
and osmium (Os), iridium (Ir), and
platinum (Pt). These elements, to-
gether with iron, cobalt, and nickel,
were formerly classed as group VIII
of the periodic table. The platinum-
group metals are relatively hard and
resistant to corrosion and are used in
jewellery and in some industrial ap-
plications (e.g. electrical contacts).
They have certain chemical similari-
ties that justify classifying them to-
gether. 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 pi-
bonding ligands. A number of com-
plexes can be made in which a hy-
drogen atom is linked directly to the
metal. The metals and their organic
compounds have considerable cata-
lytic activity. See also transition el-
ements.
Platonic hydrocarbons Hydrocar-
bons that have the same molecular
geometry as theÜve Platonic solids,
i.e. the tetrahedron, the cube, the oc-
tahedron, the dodecahedron and the
icosahedron. These hydrocarbons
have carbon atoms at the vertices of421 Platonic hydrocarbons
p