the polyhedra and single bonds along
the edges. In fact, there are only two
known Platonic hydrocarbons:
cubane (C 8 H 8 ) and dodecahedrane
(C 20 H 20 ). The hydrocarbon based on a
tetrahedron (C 4 H 4 ) does not exist be-
cause of angle strain, but substituted
derivatives C 4 X 4 are known. Angle
strain is also the reason for the
nonexistence of octahedrane (C 6 H 6 )
and icosahedrane (C 12 H 12 ).
pleochroic Denoting a crystal that
appears to be of different colours, de-
pending on the direction from which
it is viewed. It is caused by polariza-
tion of light as it passes through an
anisotropic medium.
plumbagoSee carbon.
plumbane(lead(IV) hydride)An ex-
tremely unstable gas, PbH 4 , said to be
formed by the action of acids on
magnesium–lead alloys. It wasÜrst
reported in 1924, although doubts
have since been expressed about
the existence of the compound. It
demonstrates the declining stability
of the hydrides in group 14. More sta-
ble organic derivatives are known;
e.g. trimethyl plumbane, (CH 3 ) 3 PbH.
plumbateA compound formed by
reaction of lead oxides (or hydrox-
ides) with alkali. The oxides of lead
are amphoteric (weakly acidic) and
react to give plumbate ions. With the
lead(IV) oxide, reaction with molten
alkali gives the plumbate(IV) ion
PbO 2 + 2OH–→PbO 3 2–+ H 2 OIn fact, various ions are present in
which the lead is bound to hydroxide
groups, the principal one being the
hexahydroxoplumbate(IV) ion
Pb(OH) 6 2–. This is the negative ion
present in crystalline ‘trihydrates’ of
the type K 2 PbO 3 .3H 2 O. Lead(II) oxide
gives the trihydroxoplumbate(II) ion
in alkaline solutions
PbO(s) + OH–(aq) + H 2 O(l) →
Pb(OH) 3 2–(aq)
Plumbate(IV) compounds were for-
merly referred to as orthoplumbates
(PbO 4 4–) or metaplumbates(PbO 3 2–).
Plumbate(II) compounds were called
plumbites.plumbic compoundsCompounds
of lead in its higher (+4) oxidation
state; e.g. plumbic oxide is lead(IV)
oxide, PbO 2.plumbiteSee plumbate.plumbous compoundsCom-
pounds of lead in its lower (+2) oxida-
tion state; e.g. plumbous oxide is
lead(II) oxide, PbO.plutoniumSymbol Pu. A dense sil-
very radioactive metallic transuranic
element belonging to the *actinoids;
a.n. 94; mass number of most stable
isotope 244 (half-life 7.6 × 107 years);
r.d. 19.84; m.p. 641°C; b.p. 3232°C.
Thirteen isotopes are known, by far
the most important being pluto-
nium–239 (half-life 2.44 × 104 years),
which undergoes nuclearÜssion with
slow neutrons and is therefore a vital
power source for nuclear weapons
and some nuclear reactors. About 20
tonnes of plutonium are produced
annually by the world’s nuclear reac-
tors. The element wasÜrst produced
by Seaborg, McMillan, Kennedy, and
Wahl in 1940.
A- Information from the WebElements site
PMSee particulate matter.
pnicogensSee pnictogens.pnictidesSee pnictogens.
pnictogens(pnicogens)The el-
ements of group 15 of the periodic
table, i.e. nitrogen (N), arsenic (As),
antimony (Sb), and bismuth (Bi). The
elements As, Sb, and Bi form a range
of solid-state ternary compounds
with interesting electrical and mag-423 pnictogens
p