an ill-deÜned group of metalloid el-
ements (B, Si, Ge, As, Te).
PCBSee polychlorinated biphenyl.
PCPSee phencyclidine.
peacock ore See bornite.
pearl ash See potassium
carbonate.
pearlite See steel.
pelargonic acid See nonanoic
acid.
penicillinAn antibiotic derived
from the mould Penicillium notatum;
speciÜcally it is known as penicillin G
and belongs to a class of similar sub-
stances called penicillins. They pro-
duce their effects by disrupting
synthesis of the bacterial cell wall,
and are used to treat a variety of in-
fections caused by bacteria.
401 pentavalent
p
SN3 CH
3 CH
O
O
OHNH
ROPenicillin
Penning ionizationA photochem-
ical process that produces a posi-
tively 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 acquir-
ing 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 ion-
ization was discovered by F. M. Pen-
ning in 1927.Penrose tiling A method of tiling a
two-dimensional plane using tiles
withÜvefold 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 mathe-
matician and physicist Sir Roger Pen-
rose (1931– ), who put forward this
idea in 1974, is the two-dimensional
analogue of a *quasicrystal.pentaerythritolA white crys-
talline compound, C(CH 2 OH) 4 ; m.p.
260 °C; b.p. 276°C (30 mmHg). It is
used in making the explosive pen-
taerythritol trinitrate and in produc-
ing resins and other organic
products.pentaerythritol tetranitrate
(PETN)A powerful high explosive
made from pentaerythritol,
C(CH 2 ONO 2 ) 4.
pentahydrateA crystalline hy-
drate that hasÜve moles of water per
mole of compound.pentaneA 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 per-
fume industry.
pentavalent(quinquevalent)Hav-
ing a valency ofÜve.