an ill-deÜned group of metalloid elements (B, Si, Ge, As, Te).
PCB See polychlorinated biphenyl.
PCP See phencyclidine.
peacock ore See bornite.
pearl ash See potassium
carbonate.
pearlite See steel.
pelargonic acid See nonanoic
acid.
penicillin An antibiotic derived
from the mould Penicillium notatum;
speciÜcally it is known as penicillin G
and belongs to a class of similar substances called penicillins. They produce their effects by disrupting
synthesis of the bacterial cell wall,
and are used to treat a variety of infections caused by bacteria.
401
pentavalent
p
S
N
C
H 3
C
H 3
O
O
OH
N
H
R
O
Penicillin
Penning ionization A photochemical process that produces a positively charged ion. For atoms A and
B, the process of Penning ionization
is written:
A* + B → A + B
+ + e
– ,
where A* denotes that the atom A
has absorbed a photon, thus acquiring enough energy for the process to
take place, and e
– is an electron. An
example of Penning ionization is the
ionization of mercury by argon:
Ar* + Hg → Ar + Hg
+ + e
–
,
which occurs because the energy of
the metastable state of argon is
higher than the ionization energy of
mercury. The process of Penning ionization was discovered by F. M. Penning in 1927.
Penrose tiling A method of tiling a
two-dimensional plane using tiles
with Üvefold symmetry. Since this
type of symmetry is not allowed in
crystallography, two types of tile are
needed, which are called ‘fat’ and
‘skinny’. Adjacent tiles have to obey
certain matching rules. Penrose
tiling, named after the British mathematician and physicist Sir Roger Penrose (1931– ), who put forward this
idea in 1974, is the two-dimensional
analogue of a *quasicrystal.
pentaerythritol A white crystalline compound, C(CH 2 OH) 4 ; m.p.
260°C; b.p. 276°C (30 mmHg). It is
used in making the explosive pentaerythritol trinitrate and in producing resins and other organic
products.
pentaerythritol tetranitrate
(PETN) A powerful high explosive
made from pentaerythritol,
C(CH 2 ONO 2 ) 4 .
pentahydrate A crystalline hydrate that has Üve moles of water per
mole of compound.
pentane A straight-chain alkane
hydrocarbon, C 5 H 12 ; r.d. 0.63; m.p.
–129.7°C; b.p. 36.1°C. It is obtained
by distillation of petroleum.
pentanedioic acid (glutaric acid)
A simple dicarboxylic acid,
HOOC(CH 2 ) 3 COOH; m.p. 96°C; b.p.
200°C. It is used in the production of
certain polymers.
pentanoic acid (valeric acid) A
colourless liquid *carboxylic acid,
CH 3 (CH 2 ) 3 COOH; r.d. 0.9; m.p. –34°C;
b.p. 186.05°C. It is used in the perfume industry.
pentavalent (quinquevalent) Having a valency of Üve.
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PCB See polychlorinated biphenyl.
PCP See phencyclidine.
peacock ore See bornite.
pearl ash See potassium
carbonate.
pearlite See steel.
pelargonic acid See nonanoic
acid.
penicillin An antibiotic derived
from the mould Penicillium notatum;
speciÜcally it is known as penicillin G
and belongs to a class of similar substances called penicillins. They produce their effects by disrupting
synthesis of the bacterial cell wall,
and are used to treat a variety of infections caused by bacteria.
401
pentavalent
p
S
N
C
H 3
C
H 3
O
O
OH
N
H
R
O
Penicillin
Penning ionization A photochemical process that produces a positively charged ion. For atoms A and
B, the process of Penning ionization
is written:
A* + B → A + B
+ + e
– ,
where A* denotes that the atom A
has absorbed a photon, thus acquiring enough energy for the process to
take place, and e
– is an electron. An
example of Penning ionization is the
ionization of mercury by argon:
Ar* + Hg → Ar + Hg
+ + e
–
,
which occurs because the energy of
the metastable state of argon is
higher than the ionization energy of
mercury. The process of Penning ionization was discovered by F. M. Penning in 1927.
Penrose tiling A method of tiling a
two-dimensional plane using tiles
with Üvefold symmetry. Since this
type of symmetry is not allowed in
crystallography, two types of tile are
needed, which are called ‘fat’ and
‘skinny’. Adjacent tiles have to obey
certain matching rules. Penrose
tiling, named after the British mathematician and physicist Sir Roger Penrose (1931– ), who put forward this
idea in 1974, is the two-dimensional
analogue of a *quasicrystal.
pentaerythritol A white crystalline compound, C(CH 2 OH) 4 ; m.p.
260°C; b.p. 276°C (30 mmHg). It is
used in making the explosive pentaerythritol trinitrate and in producing resins and other organic
products.
pentaerythritol tetranitrate
(PETN) A powerful high explosive
made from pentaerythritol,
C(CH 2 ONO 2 ) 4 .
pentahydrate A crystalline hydrate that has Üve moles of water per
mole of compound.
pentane A straight-chain alkane
hydrocarbon, C 5 H 12 ; r.d. 0.63; m.p.
–129.7°C; b.p. 36.1°C. It is obtained
by distillation of petroleum.
pentanedioic acid (glutaric acid)
A simple dicarboxylic acid,
HOOC(CH 2 ) 3 COOH; m.p. 96°C; b.p.
200°C. It is used in the production of
certain polymers.
pentanoic acid (valeric acid) A
colourless liquid *carboxylic acid,
CH 3 (CH 2 ) 3 COOH; r.d. 0.9; m.p. –34°C;
b.p. 186.05°C. It is used in the perfume industry.
pentavalent (quinquevalent) Having a valency of Üve.
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